core: rename vec::rev_each{,i} to vec::each{,i}_reverse

I'm making this change because the _reverse suffix is more commonly
used in libcore/libstd.
core: add char_at_reverse
core: add char_len method
core: add str::each{,i}_reverse
testsuite: Add test for #4210
core: add Reader, Writer, ReaderUtil, WriterUtil to prelude. Close #4182.
add stage3 markers where necessary for dist-snap
auto merge of #5453 : catamorphism/rust/issue-4120, r=catamorphism
auto merge of #5447 : erickt/rust/incoming, r=graydon

This normalizes the how we call reverse iteration functions. It also adds a char_len method, and a method to iterate backwards over a string.
auto merge of #5455 : pcwalton/rust/framework, r=catamorphism

r? @catamorphism
renamed str::view -> slice_DBG_BRWD
renamed str::slice -> slice_DBG_UNIQ
changed vec slice method -> to_owned()
renamed vec view method  -> slice_V_DBG_BRWD
auto merge of #5456 : graydon/rust/fixups, r=pcwalton

Stage markers for stage3 and a trivial prelude fix.
auto merge of #5458 : pcwalton/rust/valgrind-suppress, r=pcwalton
Switch Module.children from oldmap
Switch Module.import_resolutions from oldmap
auto merge of #5465 : sanxiyn/rust/remove-oldmap, r=sanxiyn

Improves resolve by 20% as measured by time-passes on rustc.
Move the function to time passes to util
Time type checking passes separately
test: XFAIL test issue-4120 due to Valgrind complaints.
removed str::slice_DBG_UNIQ