check in a bunch of stuff
README.md: rewrite
.build.yml: add a task to print coverage
add support for builds.sr.ht
posix/benchmark: add benchmark_app_proc_play

This differentiates it from benchmark_app_proc_idle, which is what the
old benchmark did, so the old benchmark gets a new name.
posix: add benchmark
posix/test: make struct literal spacing uniform
mcu: add freertos

It's not being used yet.
scripts: add debug-cross

A little script to set up a debugging session in tmux.
pure/seq: add some assertions; silence a warning
pure/seq: fix a bug in frame_index_update

Despite my attempt to avoid introducing a bug during refactoring,
I succeeded in not testing a case that differed.

This was precipiated by the insensitivity of gcovr to branches of a
conditional expression (?:). Are there any good reasons to use it?
pure/seq: use new seq_advance implementation
pure/seq: add and test seq_advance2

We want to test seq_advance but that requires refactoring. Test
that the old and new versions work identically. Then we can replace
the old with the new.
posix/test: use struct test type in errorf
posix/test: add and test close_enough function

It will be useful to test floating-point functions.
mcu: assert finite output signal
contrib/tmux-devenv: don't invoke the editor
posix/test: remove unused code
mcu: remove unused files
pure/meson.build: remove unused source files