Grammar::BNF like run-time generate methods

[dwarring]

There's already a CSS::Specification::Build.generate method, but the interface is not quite the same
as Grammar::BNF. It's restricted to slurping definitions from a file. Would be good to get the interface more compatible.

[dwarring]

Representation needs to be RakuAST

[dwarring]

Work in progress on the rakuast branch.