feat(mpsc): add `len`, `capacity`, and `remaining` methods to mpsc #81

Job	Run time
changed_paths	3s
Check no_std (alloc)	9s
Tests (stable, ubuntu-latest)	31s
Check no_std (static)	9s
Tests (stable, windows-latest)	1m 21s
Compile benchmarks	16s
Clippy check	25s
Tests (stable, macos-latest)	37s
Miri tests	54s
rustfmt	5s
models in 'mpsc_blocking'	1m 24s
model 'mpsc_send_recv_wrap''	1m 14s
models in 'mpsc_async'	3m 28s
model 'mpsc_try_send_recv''	1m 27s
models in 'thingbuf'	2m 1s
model 'mpsc_try_recv_ref''	1m 24s
models in 'util'	1m 10s
model 'mpsc_test_skip_slot''	2m 34s
model 'mpsc_async::rx_close_unconsumed''	1m 8s
model 'mpsc_blocking::rx_close_unconsumed''	1m 8s
all loom models	1s
	21m 29s

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