Define a new setting g:rust_fold

g:rust_fold allows folding to be enabled. This lets the user turn on
folding without having to define autocommands.
Set softtabstop, textwidth, and optionally colorcolumn

Setting softtabstop makes <Del> delete 4 spaces as if it were a tab.

Setting textwidth allows comments to be wrapped automatically. It's set
at 80, which is the recommended line length for Rust programs. There are
suggestions that it should be 79, but our current style guide says 80 so
that's what we're matching.

A new setting g:rust_colorcolumn sets colorcolumn as well, to +1,101.
This indicates both the textwidth and the second stricter line length of
100 that our style guide lists.