calyx-py: Separate math generator utilities

[sampsyo]

The calyx-py library contains not only the general machinery for generating Calyx programs but also several specific math-ish unit generators: gen_exp, gen_ln, and gen_msb.

Having all this together is not only weird from first principles, it also causes one real problem: the library's source-location tracking does some funky stuff with stack traces. In particular, it ignores stack frames that come from the calyx-py library itself:

calyx/calyx-py/calyx/py_ast.py

Lines 72 to 74 in 85968c4

	if not frame.filename.startswith(library_path):
	user = frame
	break

This has the weird effect that these gen_* scripts are sometimes considered part of the library and sometimes not (e.g., when they are invoked on the command line).

Let's separate this stuff out into a standalone Python package—maybe something like frontends/gen_math or whatever. One wrinkle will be updating all the dependencies that use these generators: principally, the TVM/Relay frontend.

[ayakayorihiro]

PR #2607 adds a hack around this where files in the calyx-py library that are not py_ast.py and builder.py are considered not a part of the library. But this isn't ideal, so I'll try to separate the rest into a separate Python package!