Add NewLazyLexer and MustNewLazyLexer which accept a function that
returns the rules for the lexer. This allows us to defer the rules
definitions until they're needed.
Lexers in a, g, s, and x packages have been updated to use the new lazy
lexer.
This was done to speed up incremental compilation when working on
lexers. That is, modifying a single lexer will no longer require
recompiling all lexers.
This is a (slightly) backwards breaking change in that lexers are no
longer exported directly in the lexers package. The registry API is
"aliased" at the old location.
