spv_parsed_instruction_t cleanup

spv_parsed_instruction_t cleanup

Add members:
 - words: a pointer to an array of words in the instruction,
   in host native endianness.
 - num_words: sizes the words member

Remove member:
 - offset

This simplifies clients of spvBinaryParse, because they don't
have to handle endianness translation.

Also, it makes the binary parse API more composable, allowing
for easy chaining of binary parse clients.  A binary parse client
is handed the array of words directly instead of having to reference
some external array of all the words in the SPIR-V binary.  It also
allows a binary parse client to mutate the instruction stream before
handing off to a downstream consumer.

TODO(dneto): Still need to write the unit tests for spvBinaryParse

Fixes: https://github.com/KhronosGroup/SPIRV-Tools/issues/1