Type checking of defaults

[schwern]

As of #102 defaults are type checked. This is a side effect of a fix to type check after applying a default. It's probably a waste of CPU to type check constant defaults at runtime.

1. Type check constant defaults at compile time only.
2. Type check computed defaults at runtime.

For example.

func hello(
    Str :$place //= "World",
    Str :$greeting //= greeting_for_place($place)
) {
    say "$greeting, $place!";
}

When compiled, the default for $place would be type checked because it is a constant. When run as hello() $place would not be type checked as it is using a constant default. $greeting would be type checked as it is using a computed default. If arguments are passed in for either $place or $greeting they will be type checked.

PPI can probably be used to determine if a default is a constant. Be conservative, it is better to not recognize a complicated constant than to think a computed default is constant.