In order to support unsized constants being used as parameters, the `const` struct needs to know if it is unsized (so that the parameter can be used to set the size).
Add unsized flag to param value serialization and rtlil back-/front-end.
Add cell params to `tests/rtlil/everything.v`.
Without this check it's trivially easy to crash Yosys with a tiny RTLIL input
by specifying a constant with very large width. Fuzz testers love hitting this
over and over again.
Yosys works with bison 3.0 (or newer), but not bison 2.7 (the previous
release). Ideally, we would require "3" rather than "3.0" to give a
better error message, but bison 2.3, which still ships with macOS, does
not support major-only version requirements. With this change, building
with an outdated bison yields: `frontends/rtlil/rtlil_parser.y:25.10-14:
require bison 3.0, but have 2.3`.
The only difference between "RTLIL" and "ILANG" is that the latter is
the text representation of the former, as opposed to the in-memory
graph representation. This distinction serves no purpose but confuses
people: it is not obvious that the ILANG backend writes RTLIL graphs.
Passes `write_ilang` and `read_ilang` are provided as aliases to
`write_rtlil` and `read_rtlil` for compatibility.
