Fix clippy warnings etc. (#393)

* Fix various unsafe declarations

As reported by clippy. These are all cases where a pointer argument is
dereferenced by a declared safe function. The functions are changed to be unsafe
functions. Though some of these functions are declared pub, they all seem to be
related to internal binding matters and appear to be written correctly, just
declared incorrectly, so there shouldn't be any external impact for callers
that aren't using the crocksdb API directly.

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Fix some clippy style lints

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Fix mutability of crocksdb_load_latest_options errptr

This value can be mutated on the C side. Caught by clippy.

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Cleanup some unit-return functions

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Disable a clippy lint

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* make a transmute more typesafe

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Elide a lifetime

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Remove unnecessary unsafe blocks

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Use repr(C) structs of c_void for opaque types.

These represent opaque RocksDB types. They are used behind pointers, but are
also wrapped in other types in the higher-level bindings.

These use the strategy for opaque C types described in the nomicon [1]:
but with the exception that they contain c_void instead of [u8; 0], thus
making them uninstantiable sized types instead of ZSTs.

The c_void documentation publicly recommends using the ZST pattern from the
nomicon, but in private documentation [2] warns about UB from dereferencing
pointers to uninhabited types, which these bindings do.

Additionally, these bindings wrap some these types directly (not through
pointers) and it's impossible to repr(transparent) a ZST, without which the
unsafe casts within are dubious.

[1]: https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs
[2]: https://doc.rust-lang.org/nightly/src/core/ffi.rs.html#28



Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Apply repr(transparent) to various unsafe-cast wrappers

These are wrappers of opaque FFI types, and pointers are being unsafely cast
between the two. repr(transparent) is probably needed to make this well-defined.

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Replace mem::transmute with pointer casts

Slightly more typesafe, and more explicit about what's being cast.

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Replace more mem::transmute with casts

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Remove a no-op mem::transmute

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Remove is_power_of_two

std contains a branchless bitmagic version of this function

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Add clippy makefile target

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Add clippy to CI

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Fix types in memcpy

This code is copying from c_void pointers when it should be copying
from byte pointers.

Fortunately c_void and u8 have the same size, but I don't know if that is
guaranteed.

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Update src/rocksdb.rs

Co-Authored-By: kennytm <kennytm@gmail.com>
Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Install clippy on travis

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Adjust clippy lints for CI toolchain

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* Make a typesafe cast

Signed-off-by: Brian Anderson <andersrb@gmail.com>

* rustfmt

Signed-off-by: Brian Anderson <andersrb@gmail.com>