Sort out no_std support

Nobody has claimed this yet.

Currently `parley` has a `std` feature which can be disabled, and we are paying the maintenance cost of maintaining that, but it can't actually do anything useful when that feature is disabled. I would suggest that:

- Nothing in this repo ought to be support in "no alloc" configurations
- Fontique ought to work as "font database" and support querying, manual loading of fonts, etc without std. Backends should require std as needed (probably all of them?).
- Parley ought to to fully function in "alloc but no std" mode. It ought to have no conditional code to enable this (except perhaps some conditional imports from the alloc crate). Parley will still transitively require std in some cases through it's dependencies.

No files or tests are named. Start by inspecting the parley std feature and the Fontique and backend boundaries; done means removing unsupported no-alloc expectations, enabling Fontique font-database use without std, and making Parley function in alloc-but-no-std mode.

Written by the indexing model from the issue text.