Add `condition` as a Matcher function.

This lets you embed predicates inside `from:` clauses.  This will
be useful when I add else/commit capability to from, so that we
don't try other clauses once a given `from:` matches.  This will
let us simplify a number of definitions where we need to restate
complementary versions of the same condition for flow control.
Describe `_with designated` and various expression functions.

In addition to `_with designated`, briefly wrote up arithmetic
functions, `matcher`/`condition` functions, and higher-order
`_curry` and `_apply`.