Refactor the binary parser.

Refactor the binary parser.

The binary parser has a C API, described in binary.h.
Eventually we will make it public in libspirv.h.
The API is event-driven in the sense that a callback is called
when a valid header is parsed, and for each parsed instruction.

Classify some operand types as "concrete".  The binary parser uses
only concrete operand types to describe parsed instructions.

The old disassembler APIs are moved into disassemble.cpp

TODO: Add unit tests for spvBinaryParse.