`only` methods are used where `multies` should be, preventing dispatch to Mu.* candidates

[p6rt]

Migrated from rt.perl.org#130867 (status was 'new')

Searchable as RT130867$

[p6rt]

From @zoffixznet

Some types provide methods, like .Bool, .elems, .Str that are defined for :D subtypes. However,
these methods are defined as `only` methods, not `multi`, so the dispatch for :U subtypes fails,
instead of those be handled by Mu.* methods.

I already fixed[^1] a bunch for Baggy role, but I notice other types have similar issue​:

  <Zoffix> m​: Seq.Bool
  <camelia> rakudo-moar dd4dfb​: OUTPUT​: «Invocant requires an instance of type Seq, but a type object was passed. Did you forget a .new?␤ in block <unit> at <tmp> line 1␤␤»

We need to check the codebase for more of these. I think some construct similar to `for Any.^methods(​:all)».name -> $name { for @​all-core-types { ."$name"() }}`
can be used for automated finding of methods that need to be changed to multies.

[1] rakudo/rakudo@e8af855

[p6rt]

From @zoffixznet

Another piece-meal fix​: for Bool​:U.Str​:
rakudo/rakudo@3de5fb24c9
Raku/roast@de5f00b909

[p6rt]

From @zoffixznet

Another piece-meal fix​: for Bool​:U.Str​:
rakudo/rakudo@3de5fb24c9
Raku/roast@de5f00b909

[p6rt]

From @zoffixznet

Seq.Int, Seq.Numeric, Seq.elems are another bunch.

We need some programmatically-assisted way to audit the entire codebase for these.