Add formatted Printable interpolator `cf` (#2528)

This is a formatted version of the p"..." interpolator analogous to
Scala's f"..." interpolator. The primary difference is that it supports
formatting interpolated variables by following the variable with
"%<specifier>". For example:

printf(cf"myWire = $myWire%x\n")

This will format the hardware value "myWire" as a hexidecimal value in
the emitted Verilog. Note that literal "%" must be escaped as "%%".

Scala types and format specifiers are supported and are handled in the
same manner as in standard Scala f"..." interpolators.