*: improve build process and update rocksdb (#243)

Some trivial changes are applied:
- Refactor Makefile
- Refactor README.md
- Add the rust-toolchain
- Update RocksDB submodule
- Reorganize the tests structure

.gitmodules

-[submodule "librocksdb_sys/rocksdb"]
+[submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
 	url = https://github.com/pingcap/rocksdb.git
 	branch = release-5.15
+	shallow = true

Cargo.toml

@@ -19,10 +19,6 @@ valgrind = []
 portable = ["librocksdb_sys/portable"]
 sse = ["librocksdb_sys/sse"]
 
-[[test]]
-name = "test"
-path = "tests/test.rs"
-
 [dependencies]
 libc = "0.2.11"
 crc = "1.2"

Makefile

-.PHONY: all format
-
-# format rust code using the specified command for the specified file or directory.
-# $(call do-format-with-cmd,cmd,file-or-dir)
-define do-format-with-cmd
-	$1 --write-mode diff $2 |  grep -E "Diff .*at line" > /dev/null && $1 --write-mode overwrite $2 || exit 0
-endef
-
-# format rust code in the specified file or directory
-# a file of rust code follows the convention of having suffix '.rs'
-# $(call format-code-in,file-or-dir)
-define format-code-in
-	$(if $(filter %.rs, $1),  \
-	$(call do-format-with-cmd, rustfmt, $1), \
-	cd $1 && $(call do-format-with-cmd, cargo fmt --))
-endef
-
 all: format build test
 
+format:
+	@cargo fmt --all
+	@librocksdb_sys/crocksdb/format-diff.sh > /dev/null || true
+
 build:
 	@cargo build
 
 test:
 	@export RUST_BACKTRACE=1 && cargo test -- --nocapture
 
-format: 
-	@$(call format-code-in, .)
-	@$(call format-code-in, tests/test.rs)
-	@$(call format-code-in, librocksdb_sys)
-	# User may not install clang-format-diff.py, ignore any error here. 
-	@librocksdb_sys/crocksdb/format-diff.sh > /dev/null || true 
-
 clean:
 	@cargo clean
 	@cd librocksdb_sys && cargo clean
 
-update-rocksdb:
-	@git subtree pull -P librocksdb_sys/rocksdb https://github.com/pingcap/rocksdb.git release-5.15 --squash

README.md

-rust-rocksdb
-============
+# rust-rocksdb
 
 This library has been tested against RocksDB 5.15 on Linux and macOS.
 
-### status
+## Status
   - [x] basic open/put/get/delete/close
   - [x] rustic merge operator
   - [x] write batch (thanks @dgrnbrg!)
@@ -28,25 +27,24 @@ This library has been tested against RocksDB 5.15 on Linux and macOS.
 
 Feedback and pull requests welcome! If a particular feature of RocksDB is important to you, please let us know by opening an issue, and we will prioritize it.
 
-###### Prerequisite: RocksDB
+## Build
 
-First, use your system's package manager to install snappy.  This is optional, but lets rocksdb take advantage of better compression, and some code may require it.
-
-To install rocksdb, please refer to it's installation guide. For Windows users, please make sure you configure rocksdb with `CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS` enabled.
-
-Or enable feature `static-link`, the crate will download and complie RocksDB automatically, including its dependencies.
-
-```bash
-cargo build --features static-link
+```
+$ git submodule update --init --recursive # if you just cloned the repository
+$ cargo build
 ```
 
-### Running
+## Running
+
 ###### Cargo.toml
+
 ```rust
 [dependencies.rocksdb]
 git = "https://github.com/pingcap/rust-rocksdb.git"
 ```
+
 ###### Code
+
 ```rust
 extern crate rocksdb;
 use rocksdb::{DB, Writable};
@@ -65,6 +63,7 @@ fn main() {
 ```
 
 ###### Doing an atomic commit of several writes
+
 ```rust
 extern crate rocksdb;
 use rocksdb::{DB, WriteBatch, Writable};
@@ -83,6 +82,7 @@ fn main() {
 ```
 
 ###### Getting an Iterator
+
 ```rust
 extern crate rocksdb;
 use rocksdb::{DB, Direction, IteratorMode};
@@ -112,6 +112,7 @@ fn main() {
 ```
 
 ###### Getting an Iterator from a Snapshot
+
 ```rust
 extern crate rocksdb;
 use rocksdb::{DB, Direction};
@@ -125,6 +126,7 @@ fn main() {
 ```
 
 ###### Rustic Merge Operator
+
 ```rust
 extern crate rocksdb;
 use rocksdb::{Options, DB, MergeOperands, Writable};
@@ -162,7 +164,9 @@ fn main() {
 ```
 
 ###### Apply Some Tunings
+
 Please read [the official tuning guide](https://github.com/facebook/rocksdb/wiki/RocksDB-Tuning-Guide), and most importantly, measure performance under realistic workloads with realistic hardware.
+
 ```rust
 use rocksdb::{Options, DB};
 use rocksdb::DBCompactionStyle::DBUniversalCompaction;

rocksdb @ 84eaf9c2

-Subproject commit adf83d4ce771ea188292e4da427eafd2667b9338
+Subproject commit 84eaf9c23a1a277835608dcb979ae97229391981

rust-toolchain

+stable

tests/test.rs → tests/cases/mod.rs

-extern crate byteorder;
-extern crate crc;
-extern crate rand;
-extern crate rocksdb;
-extern crate tempdir;
-
 mod test_column_family;
 mod test_compact_range;
 mod test_compaction_filter;

tests/test_event_listener.rs → tests/cases/test_event_listener.rs

@@ -11,11 +11,11 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use cases::test_ingest_external_file::gen_sst;
 use rocksdb::*;
 use std::sync::atomic::*;
 use std::sync::Arc;
 use tempdir::TempDir;
-use test_ingest_external_file::gen_sst;
 
 #[derive(Default, Clone)]
 struct EventCounter {

tests/tests.rs

+extern crate byteorder;
+extern crate crc;
+extern crate rand;
+extern crate rocksdb;
+extern crate tempdir;
+
+mod cases;