(maint) Fix Pattern / Variant Type assignability

This fix corrects Pattern.assignable?(Variant) which was not
correctly supported in the type calculator. (It gave up if t2
was not a String or an Enum). Now, if t2 is a Variant,
the check recurses to the types described by the enum.
(maint) Fix Enum / Variant assignability

The type calculator did not correctly compute assignability
for an Enum when assigning a Variant type. Now, all variants
are checked if they are assignable to the enum type.
(maint) Fix problem with Enum subset not matching Enum

This corrects a problem where Enum[a] was not assignable to
Enum[a,b], since it is a subset, it should be assignable.
(maint) Add the ability to inject calling scope into function.

The InteralFunction API now has the ability to "inject" and weave
the calling scope into the dispatched arguments by calling
the method scope_param() when building the dispatcher.
(PUP-2845) Move the epp function to the 4x function API.

This moves the epp function to the new function API.
The type checking is much approved as a consequence as the function
is now protected against spilling non variable name compliant
variables into the scope.
(PUP-2845) Move the inline_epp function to the new function API.

This moves the function inline_epp to use the new function API.
This also improves the argument checking since it is not not possible
to leak non variable name compliant variables into the scope.
(PUP-2845) Remove comment about skipping mapping in call_function

The comment refers to removed behaviour. Now, since the epp
functions are moved, there is also no need to map them (there are PRs in
flight that will add such behavior).
Merge pull request #2815 from hlindberg/PUP-2845_move_epp_functions

(PUP-2845) Move epp functions