LinearMap is quite a bit faster, and is fully owned/sendable without
requiring copies. The older std::map also doesn't use explicit self and
relies on mutable fields.
needed.
Regarding soundness: there was a subtle bug in how it was done before; see the
compile-fail test for an example.
Regarding reborrowing: reborrowing allows mut and const
slices/borrowed-pointers to be used with pure fns that expect immutable data.
r=brson
