the current hack to emulate lexical exporting has edge cases

[doy]

Given this situation:

# Foo/Bar/Baz.pm
package Foo::Bar;
use mop;
class Baz extends Foo::Bar { }
no mop;

# Foo/Bar.pm
package Foo;
use mop;
class Bar {
    method class { ... }
}
no mop;

# script.pl
use Foo::Bar::Baz;

what currently happens is:

• the mop keywords are exported into Foo::Bar
• Baz starts parsing
• as part of the parsing, Foo::Bar is loaded
• the mop keywords are exported into Foo
• Bar starts parsing
• Bar's FINALIZE installs class into Foo::Bar as a method
• the mop keywords are removed from Foo
• Baz finishes parsing, and generates an OP_ENTERSUB, pointing to Foo::Bar::class
• Foo::Bar::Baz enters runtime, and calls Foo::Bar::class the method, not the keyword
• the method then does something random (probably dies)

This could be worked around once we rewrite the parser in XS (#93) by giving the keywords a call checker that optimizes out the entersub op, but hopefully we can get lexical exports (#99) working before that's necessary.