MPack v0.8.2

Changes:

• Fixed incorrect element tracking in mpack_write_tag()
• Added type-generic writer functions mpack_write() and mpack_write_kv()
• Added mpack_write_object_bytes() to insert pre-encoded MessagePack into a larger message
• Enabled strings in all builds by default
• Fixed unit test errors under -ffast-math
• Fixed some compiler warnings