Constructors: mapping/ordered, mapping-unfold/ordered

These are the same as mapping and mapping-unfold, except that it is an error if the keys are not in order, and they may be more efficient.

Copying and conversion: alist->mapping/ordered, alist->mapping/ordered!

These are the same as alist->mapping and alist->mapping!, except that it is an error if the keys in the alist are not in order, and they may be more efficient.

These should provided in both (srfi 146) and (srfi 146 hash), but probably don't do anything special in (srfi 146 hash).

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