The idea being that *Raku* should provide a HLL interface to obtain
a Sub object for the given method name on the invocant.

    $ raku -e 'say Int.Sub("Str")(687, :subscript)'
    ₆₈₇

In place of the .^find_method logic, which is *not* Raku specific
according to the documentation:

  https://docs.raku.org/type/Metamodel/DefiniteHOW#method_find_method

- use nqp::tryfindmethod instead of nqp::findmethod
- use nqp::ifnull instead of // to handle VMnull values

In hindsight, this is probably better, as .Sub may be considered
premature huffmanization.  And what is being returned, is usually
*not* a Sub object anyway.

As suggested by vrurg++