Switch spirv-tools to Rust-GPU/SPIRV-Tools git dependency

Switch from crates.io spirv-tools 0.13.0 to the Rust-GPU/SPIRV-Tools
repository via git dependency. This provides access to the new pure-Rust
SPIR-V validator and egglog-based optimizer.
When using as a git dependency, set SPIRV_HEADERS_PATH to point to a
local SPIRV-Headers checkout since the submodule isn't available.

Author: LegNeato