Replaces the use of spirv-tools' binaries (as, opt, and val) with the ability use the compiled C++ code instead, or in addition to the spirv binaries.

Additions

• Adds spirv-headers and spirv-tools as git submodules (will add docs about retrieiving/updating them for contributors)
• Adds spirv-tools-sys which builds spirv-tools, including a small generator script that generates some includes based on spirv-headers
• Adds a spirv-tools-sys/src/c/opt.cpp which is just a thin (and incomplete) C wrapper around Optimizer because C++
• Adds a spirv-tools crate which provides a slighly nicer API on top of spirv-tools-sys

Changes

• Replaces the usage of the spirv-opt binary in link.rs with spirv_tools::opt::Optimizer
• Replaces the usage of spirv-val for validating resulting binaries in link.rs with spirv_tools::val::Validator
• Replaces the usage of spirv-as in the linker tests with spirv_tools::as::Assembler
• All of the crates that use rustc_codegen_spirv now have a use-installed-tools and use-compiled-tools feature flags, where use-compiled-tools is the default feature for optimal user experience.

Missing

• There is one change I had to do in our spirv-tools fork due some incredible strangeness with libstdc++ streams, so right now the Assembler will choke on hexadecimal floats, but we only use the Assembler in tests and none of them currently use hexadecimal floats, so figured this would be ok for now, the issue is fixable but doing C++ code was making me sad so I punted until it became an actual issue.

Resolves: #31