Have `hdl21.pdk.compile()` return the stuff it compiled

[dan-fritchman]

hdl21.pdk.compile() has this call-signature:

def compile(src: Elaboratables) -> None

In other words, "take some Module(s) ("Elaboratables") and modify them inline; return nothing". This signature is enforced for any compatible PDK-package.

It's pretty frequent we want to nest this among other manipulations, such as:

    @hs.sim
    class MySim:
        tb = h.pdk.compile(MyTbGenerator(my_params)) # <= here

The issue: make compile return the stuff it's input, in "the same shape" that it came in.
The Walker each PDK uses internally already does this, and should serve as a fine example. In fact, I expect all PDK-packages will just return via a Walker method directly.

The most hassle here is just updating the existing PDK packages. The in-source ones (sample, sky130, asap7) are easy enough; others that live in private places need a heads-up. I believe @aviralpandey has a few of these that would be effected, and is welcome to weigh in - especially if such a change wouldn't be welcome.