[Libre-soc-dev] naming change in nmutil

[Luke Kenneth Casson Leighton]

https://libera.irclog.whitequark.org/nmigen/2021-08-24#30728292;

a discussion there, prompted me to do a global search/replace
of the naming of ready/valid in the StageAPI.  i have of course
just realised that data_i and data_o need the same treatment.

apparently nmigen gained a convention to place o_ and i_
at the front of signals since the time that nmutil's StageAPI
was written.  on balance it's a good idea to follow that convention.

l.

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