This adds a `precompile_validator_lookup` flag (defaulting to false) to
`use Vex.Struct`.

This changes behavior as follows:
* a module attribute (@precompiled_validator_lookup) is initialized for the struct that holds a lookup data structure
* calls to `validates :attribute, <validations>` proactively look up the
validator AT COMPILE TIME and add them to the lookup data structure
* At runtime, the expensive validator lookup is then skipped in favor of just performing a Map fetch from the precompiled
validator lookup datastructure.

It is allowed to state `validates :attribute, &myfunction/1`.
This special case is expanded to `validates :attribute, by:
&myfunction/1` during the validation at runtime.

Therefore, we need to cache the `:by` validator if we encounter a
function.