Handle region errors where the actual scope is a statement

Prevents a case of "unknown scope, this is a bug".

I judged this too small to review. Closes #4500
Fix the difference method on bit vectors
testsuite: Add license
testsuite: Add xfailed test for #4335
Merge pull request #4546 from alexcrichton/bitv-difference-bug

Fix the difference method on bit vectors
Add Alex Crichton to AUTHORS
repr: represent char values as char literals rather than integers.
add a Set trait and implement it for TreeSet
add a LinearSet type (implementing the Set trait)
Merge pull request #4551 from SimonSapin/char-repr

repr: represent char values as char literals rather than integers.
Add Simon Sapin to AUTHORS
Merge pull request #4554 from thestinger/set

add a Set trait