Clean up skolemization with placeholder

[csmoe]

cc rust-lang/rust#54649

EDIT (mark-i-m): adding some instructions:

• We are replacing the terminology "skolemization" with "placeholder" in the compiler.
• We would like to do the same in the guide to stay consistent. We would like to replace "skolemization", "skolemize", "skolemized", etc appropriately.

[mark-i-m]

cc #210

[mark-i-m]

@csmoe Does this also apply to the old traits system too?

[csmoe]

@mark-i-m yes, all the things are "placeholder-fy"ed.