feat(mpsc): add `len`, `capacity`, and `remaining` methods to mpsc #80

Job	Run time
changed_paths	3s
Compile benchmarks	16s
rustfmt	6s
Clippy check	17s
Check no_std (alloc)	9s
Miri tests	46s
Check no_std (static)	11s
models in 'mpsc_blocking'	1m 25s
model 'mpsc_send_recv_wrap''	1m 17s
models in 'mpsc_async'	3m 33s
model 'mpsc_try_send_recv''	1m 25s
Tests (stable, ubuntu-latest)	31s
models in 'thingbuf'	1m 56s
model 'mpsc_try_recv_ref''	1m 30s
Tests (stable, windows-latest)	1m 53s
models in 'util'	1m 7s
model 'mpsc_test_skip_slot''	2m 42s
Tests (stable, macos-latest)	1m 4s
model 'mpsc_async::rx_close_unconsumed''	1m 11s
model 'mpsc_blocking::rx_close_unconsumed''	1m 7s
all loom models	1s
	22m 30s

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