Compare file:line prefix case-insensitively on win32
Paths are case insensitive on windows and rustc and compiletest may disagree on casing.
Fixes test compile-fail/circular_modules_main
Added %p directive to fmt!, which expects *T as argument
auto merge of #8085 : mrordinaire/rust/percent-p, r=huonw

pull request for #8011
auto merge of #8003 : crnobog/rust/case-insensitive-error-prefix, r=cmr

Paths are case insensitive on windows and rustc and compiletest may disagree on casing.
Fixes test compile-fail/circular_modules_main on win32
Added str::char_offset_iter() and str::rev_char_offset_iter()
Renamed bytes_iter to byte_iter to match other iterators
Refactored str Iterators to use DoubleEnded Iterators and typedefs instead of wrapper structs
Reordered the Iterator section
Whitespace fixup
Moved clunky `each_split_within` function to the one place in the tree where it's actually needed
Replaced all block doccomments in str with line doccomments