This commit:
* moves lists of universally useful imports from `nmigen` to
`nmigen.hdl` and `nmigen.lib`, reimporting them in `nmigen`;
* replaces lots of imports from individual parts of `nmigen.hdl`
with a star import from `nmigen.hdl`;
* replaces imports in tests with what we expect downstream code
to use;
* adds some missing imports in `nmigen.formal`.
Although a dir="oe" pin is generally equivalent to dir="io" pin with
the i* signal(s) disconnected, they are not equivalent, because some
pins may not be able to support input buffers at all, either because
there are no input buffers, or because the input buffers are consumed
by some other resource.
E.g. this can happen on iCE40 when the input buffer is consumed by
a PLL.
Without this register, static hazards in the encoder could cause
multiple encoder output bits to toggle, which would be incorrectly
sampled by the 2FF synchronizer.
Reported by @Wren6991.
The current behavior was introduced in 65702719, which was a wrong
fix for an issue that was actually fixed in 12e04e4e. This commit
effectively reverts 65702719 and 1782b841.
