Implement std::num::Zero for bool
Closes #8024.
auto merge of #8026 : poiru/rust/issue-8024, r=alexcrichton

Closes #8024.
debuginfo: Implemented support for destructured locals.

Conflicts:
	src/librustc/middle/trans/debuginfo.rs
	src/test/debug-info/destructured-local.rs
debuginfo: Support for function arguments. (WIP)
debuginfo: Support for destructured function arguments.
debuginfo: Minor cleanup now possible with new snapshot.
debuginfo: Fixed a few things for PR.