Avoid creating a new array on every Lexer.all call

[ashmaroli]

• Memoizing Lexer.all since every call to registry.values.uniq will return a new array for the same contents
  (same #hash value but different #object_id values)

Note: This is under the assumption that the Lexer.all list shouldn't change for the lifetime and all user-defined lexers have been registered by this point.

[jneen]

I am happy with this as long as we invalidate the cache when registering new lexers. Users often define lexers for custom languages, and we want those to show up in the registry and be accessible for guessing.