[new AA] core.internal.hash fixes

[IgorStepanov]

This PR contains the follownig changes:

1. some hashOf functions has unmarked with @trusted nothrow: we can't guarantee that struct hashOf is @trusted or nothrow.
2. typedef support added: current AA implementation supports typedef keys (deprecated but allowed), and dmd tests checks that typedef keys are allowed.
3. typeof(null) case for hashOf added.
4. seedHash has unmarked with private, because this function is usefull for hash chaining inside user toHash
5. if struct has toHash member, but it cannot be called with passed this hashOf writes warning, but doesn't raise error. (for backward compatibility: current AA implementation ingores wrong toHash in this case)
6. pass value by ref into foreach in hashOf for AA: avoid to call postblit.

[DmitryOlshansky]

I take it that being a template hashOf relies on deduction? Is it necessary to keep commented out attributes?

[IgorStepanov]

Hmm... Yes. This comments isn't necessary. I'll remove them.

[IgorStepanov]

Done.

[DmitryOlshansky]

Ehm? Shouldn't we just remove or fix it?

[IgorStepanov]

Ok, I'll remove this comment.

[IgorStepanov]

Done

[DmitryOlshansky]

Looks fine, I'd rather wait on one of druntime guys to take a look though ;)

P.S. Any unittests for this stuff?

[IgorStepanov]

I've add test for typeof(null);
I don't want to add typedef tests, because typedef is deprecated.

[IgorStepanov]

@MartinNowak ping

[IgorStepanov]

Thanks!