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
Moved all numeric modules in core into own directory

Reason: Better grouping of related modules, future-proving for a more extensive math library.
Added Round trait to core

It contains the functions floor(), ceil() and fract(), and is implemented
for all numeric types (on integers they just return self and 0).