Convert all uses of vec::slice to vec::view Issue #3869

Issue #3869
review? @nikomatsakis 

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.