Issue 2442 - opApply does not allow inferring parameter types when overloaded on const

[yebblies]

Improve opApply type deduction to include function modifier matching, and allow selecting the best match when modifier overloads exist.

This allows you to use type inference in foreach while having different versions of opApply for mutable, const, immutable etc.

http://d.puremagic.com/issues/show_bug.cgi?id=2442

[9rnsr]

Some of my thoughts.

1. Ambiguous matching causes incorrect overload resolution [BUG].
   Two or more matchings should cause error, but your code selects incorrect one opApply.
2. Reduce code duplication
   inferApplyArgTypesY and queryApplyArgTypesY are similar. They should be merged.
3. More clear test code
   The result of overload resolution is not clear. Set global flag that means which opApply is really selected, and assert it.
   And then, move test code file from compilable to runnable.
4. Improve code for merging my pull request [important to me]
   Now codes more mergable with dmd/pull/71.

My fixes based on your changes are here.
https://github.com/9rnsr/dmd/commits/fix2442

[yebblies]

I'll comment on the individual commits in your branch.

[WalterBright]

Superseded by #581