Deprecate the use of `Signal.attrs` mapping to specify platform-specific attributes

[whitequark]

Amaranth has a goal of not performing any platform-specific decisions until the moment when elaborate is called, and keeping the object tree intact (not mutated) during/after elaborate. However, the current design of attrs makes this impossible: anything that goes inside is by necessity platform-specific, and so it must be mutated within elaborate.

Amaranth's platform code is one of the worst offenders here, since add_clock_constraint usually adds a (platform-specific!) keep attribute to the clock.

Instead, I propose that attributes are added to signals using an API like:

platform.add_attribute(sig, "keep")
platform.add_attribute(sig, "dont_touch", "TRUE")

This way, the platform-specific attributes are kept inside the platform, and applied late in the compilation pipeline (during NIR generation or NIR to RTLIL/Verilog conversion, probably).

This also opens the opportunity to make signals almost entirely immutable, finishing the work started in #1067.

[whitequark]

Oh, I forgot that #1466 already exists, my bad.