Motivation
* Other libraries often have byte buffer storage with similar concepts as
NIO's `ByteBuffer`, linear storage with reader and writer indices - it would
be nice to have efficient conversion to and from these objects.
* Efficienct conversion would likely mean taking ownership of the byte
storage allocated by the other object and exposing `ByteBuffer`s
internals so that they may be owned by another object, there is no API
for this currently.
Modifications
* Added new methods behind `CustomByteBufferAllocator` SPI.
* Added public `ByteBuffer` initializer for adopting externally-allocated memory
* Made `ByteBufferAllocator` initializer public with custom `malloc`/`realloc`/`free`/`memcpy` hooks
* Updated `realloc` signature to pass old capacity alongside new capacity to allow implementations to
copy over bytes
* Added `withVeryUnsafeMutableBytesWithStorageManagement` for mutable storage access
* Updated all existing tests using internal allocator API to use the new API
* Added tests for custom allocator functionality
Result
* Custom allocators can manage `ByteBuffer` memory with full size information
* External systems can transfer memory ownership to `ByteBuffer`
* System allocator maintains zero overhead due to inlining of the wrapper
* API (SPI) is extensible for future allocation system integrations
Rick Newton-Rogers
661bfc5aea