Replace "inline" with OPUS_INLINE.

Newer versions of MSVC are unhappy with the strategy of the build
 environment redefining "inline" (even though they don't support the
 actual keyword). Instead we define OPUS_INLINE to the right thing
 in opus_defines.h.

This is the same approach we use for restrict.
Add a little missive when compiling without optimization.

The library really depends on all the little fixed point math functions
 being inlined in order to get acceptable performance. It turns out that
 it's very easy for someone to compile with optimization disable when
 twiddling cflags or cooking up their own build system.
Makes stereo savings still less aggressive

Caps the savings at 1 bit per (coded) sample. This doesn't really increase
the bitrate and fixes some issues, e.g. with IgorC's "twilight" sample.
Remove now-unused check_decoder_option.
Avoid a bogus uninitialized warning and simplify some code.
Implements OPUS_RESET_STATE for multi-stream encoder
Moves opus_packet_parse_impl() from opus_decoder.c to opus.c

Because it's indirectly used in the encoder (through the repackerizer).
Fixes DISABLE_FLOAT_API build
Removes a float var that shouldn't have been there

Defining FIXED_POINT and DISABLE_FLOAT_API now leaves no float instruction