When we check two type objects equality, we should use Type::equals instead of directly pointer comparison.
Default args and arg identifiers are now volatile information in TypeFunction.

1. The extra informations would be kept after semantic analysis for function pointer and delegate types. But merging type would strip all the extras from the type structure, then cache the stripped copy into.

2. The types has extra informations cannot be named. In other words, AliasDeclaration would always strip the aliased type. This rule also works for Template Type Parameter, and fixes issue 3646 well.

3. CondExp would strip them from the type of its result.
Improved fix for Issue 3866 - anonymous delegate with default parameters cross-talks to another anonymous delegate

This reverts commit acc22ce25db42facfe4917aeceabd28a410f4c95,
and moves original test into runnable/functype.d
fix Issue 3646 - Default values of function arguments are ignored when instantiating a template.
fix Issue 8579 - Default parameter appears a part of typeof().stringof of a function variable