This reverts commit 4e5f0bc961.
Unfortunately, in std contexts, the `alloc` crate is not imported. That
means we cannot rely on it being accessible and it in fact breaks builds
of crates which are not `no_std`.
Fixes#155.
This decreases indentation levels on the various implemenations, it
groups the implementations together physically, and (spoiler alert!)
we'll actually need the dyn Field trait object-ness (much) later on.
