Issue the correct deref count for the implicit deref that occurs
as part of an autoslice operation (#6272).

Fixes #6272.
Remove remnants of mutable fields from mem_categorization, borrowck.
auto merge of #6373 : nikomatsakis/rust/issue-6355-perf-regression, r=graydon

Fix #6355 and #6272---we were not giving the correct index to the derefs that occur as part of the rooting process, resulting in extra copies and generally bogus behavior. Haven't quite produced the right test for this, but I thought I'd push the fix in the meantime. Test will follow shortly.

r? @graydon