Eliminate the notion of "bound resolution" from inference.md.

[stereotype441]

In its place, the spec's definition of "T_0 bounded" is tightened up so that for any type T, there is exactly one type T_0 such that T is T_0 bounded. Then, instead of saying "let U_0 be the bound resolution of T_0", we can say "let U_0 be the unique type such that T_0 is U_0 bounded."

This avoids some redundancy, since we no longer need to define separate but related notions of "T_0 bounded" and "bound resolution".

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