Convert all uses of vec::slice to vec::view Issue #3869
Rename const_view to const_slice
Renamed mut_view to mut_slice
Fix windows build error.  `buf` is borrowed by the call to
`as_mut_buf()` and so we must invoke `slice()` outside of that
call.
auto merge of #4939 : nickdesaulniers/rust/issue3869, r=nikomatsakis

review? @nikomatsakis
Issue #3869