serial: migrate to lib.wiring components.

[jfng]

As a breaking change, divisor_bits must now be greater than or equal to bits_for(divisor).

[whitequark]

I'm a bit wary of merging this--we do need to define a (new, though arguably there wasn't one) process for adding peripherals to amaranth-stdio and that would likely change AsyncSerial a fair bit.

[jfng]

I'm a bit wary of merging this--we do need to define a (new, though arguably there wasn't one) process for adding peripherals to amaranth-stdio and that would likely change AsyncSerial a fair bit.

I also believe that AsyncSerial will see significant changes (i.e. be rewritten) as we figure out our strategy for -stdio.

The motivation of this PR is to make AsyncSerial useful for experimenting with component metadata and SoC peripherals; I don't really need it merged, but doing so would be convenient.

[whitequark]

LGTM with the caveat mentioned.

[jfng]

Thanks for reviewing!