fix a few issues preventing no-std builds from ... building

this includes a `Makefile` that exercises the various crate configs.
most annoyingly, several doc comments needed to grow
`#[cfg(feature="fmt")]` blocks so docs continue to build with that
feature enabled or disabled.

carved out a way to run exhaustive tests; they should be written as
`#[ignore]`, and then the makefile will run even ignored tests on the
expectation that this will run the exhaustive (but slower) suite.
exhaustive tests are not yet written. they'll probably involve spanning
4 byte sequences from 0 to 2^32-1.

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