Remove support for the internal SPIR-V text format

The translator currently supports its own textual format for SPIR-V assembly. This format is incompatible with the official format handled by the SPIRV-Tools. It is furthermore poorly tested, probably doesn't support all the corner cases and is generally a liability to the translator code base.

The translator currently produces or consumes this format via three different options:
- `--spirv-text` for a translation from IR to the SPIR-V text format
- `--to-binary` to assemble a SPIR-V text file to a SPIR-V binary
- `--to-text` to disassemble a SPIR-V binary to a SPIR-V text file

I'm tempted to suggest that none of this functionality belongs in the translator and that we should get rid of it entirely, using the relevant SPIR-V Tools for testing.

A possible staged approach could be to:
- [ ] Require SPIR-V Tools as a hard dependency and remove the optionality of SPIR-V validation (correctness shouldn't be an option :)) in the testing.
- [ ] Remove support for `--to-text` and use `spirv-dis`. There are 11 uses in tests.
- [ ] Remove support for `--to-binary` and use `spirv-as`. There are 17 uses in tests.
- [ ] Remove support for `--spirv-text` completely (125 uses in tests) and use `spirv-dis` in testing. The various name map data structures could be kept for in-debugger use (I know @neildhickey has been thinking about this).

These changes would make it easier to reuse code from SPIR-V tools as suggested in #32.

Start by tracing the three options named in the issue: --spirv-text, --to-binary, and --to-text, then inspect their 11, 17, and 125 test uses. Review the proposed SPIR-V Tools replacements, including spirv-dis and spirv-as, and determine whether the testing dependency should become mandatory. Done means the internal text-format paths and their test usage are removed without leaving the translator dependent on them.

Written by the indexing model from the issue text.