Feature/lambda

[joaquintides]

Closes #93. Points for discussion:

• Currently, mp_bind expressions are opaque to mp_lambda. For instance, mp_lambda<mp_bind<std::pair, _1, _2>>::fn<char, int> evaluates to mp_bind<std::pair, _1, _2>. We may decide to see through mp_bind and related utilities (my personal inclination is to leave it as is).
• Independently of the above, we can provide mp_protect (mp_quote is taken) so that, for instance, mp_lambda<mp_protect<_1>>::fn<int> evaluates to _1.

[pdimov]

Why are you making X(_1) "work" for _1=void? There's no X(void) type, that's a C-compatible way to spell X(). This shouldn't work.

[pdimov]

See https://godbolt.org/z/Pzssh44xa

[joaquintides]

Yes, you're right, I added that as a sort of extension via lambda_devoid_args. I can easily remove it if so you wish.

[pdimov]

I so wish, yes.