Move version detection to runtime

Avoids the need to wait for compilation of a build script.

Caches the result of running rustc so that multiple build scripts will use the same result.

Support Rust 1.31

• Deprecate date! macro, because it can't be implemented on Rust 1.31
• Run tests on MSRV (previously only ran cargo check)
• Use cargo rdme to sync README.md with crate docs.
• Use github actions to run lychee link-checker on README.md

This reduces the MSRV from 1.32 to 1.31.

Deprecate `spec!` macro in favor of helper methods

Use the RustVersion.is_since_major_version and RustVersion.is_since_patch_version methods instead of is_since(spec!(...)).

This avoids a macro import, is simpler, faster, and allows const evaulation.

The macro will be removed in a future version.

Fix compilation on MSRV

• Bump MSRV from 1.31 to 1.32 because of needed macro features
• Add basic compile-time benchmarks (only one laptop)
• Fix broken documentation link to StableVersionSpec::minor
• Fix reference to rustversion crate in README.md

Initial release as rustversion-detect

Code based off rustversion, but with all procedural macro code removed.