move rustc_type_ir Term things to term_kind.rs

[khyperia]

• TyKind, AliasTy, and AliasTyKind live in ty_kind.rs
• ConstKind, UnevaluatedConst, and UnevaluatedConstKind live in const_kind.rs
• TermKind, AliasTerm, and AliasTermKind live in... generic_arg.rs and predicate.rs ???

create a new file, term_kind.rs, that holds TermKind, AliasTerm, and AliasTermKind

(note that Ty, Const, and Term live in rustc_middle, not rustc_type_ir)

[rustbot]

r? @dingxiangfei2009

rustbot has assigned @dingxiangfei2009.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler, types
• compiler, types expanded to 73 candidates
• Random selection from 18 candidates