Investigate using Mouse XS accessors, if available at runtime

[schwern]

When it comes to setters with simple type checks, Mouse is by far the fastest. We're talking 10x faster. Mouse's getters are about 3-4x faster (Moo's could be as fast).

See https://gist.github.com/schwern/10590432 for accessor benchmarks.

• Investigate whether we can make use of Mouse XS accessor generation at runtime.
• Check that it compiles significantly faster than just using Mouse.