variant macro for blocks

[Lokathor]

I often want to cfg some code, but I want to do it on blocks inside a function instead of at the item level.

Would you be open to adding a second macro such as cfg_if_block (or other name) that does all the same stuff but working on :block instead of :item? I can do the PR if you're interested in the idea.

[gnzlbg]

Possibly dup of #8 .

[Lokathor]

I already have a PR waiting for the docs related issues.

[Lokathor]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f26008f95e5aebf5e8276fa804940a51

demo attempt. It works in some examples, but may have missed some sort of edge case.

EDIT: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=eee7c706369e151473dfbd79ca69d02d has some test usages, and seems to work in all four cases.

[alexcrichton]

Given @ogoffart's implementation I think it would be easy enough to switch to :tt lists and use the same macro for supporting items/expressions/etc, probably even methods!

[alexcrichton]

Fixed in #28 I believe