v2.0 - Add tests asserting curried methods are actually curried #124

Currying was converted to idiomatic form across the library in #40, but nothing
asserts that the "curried" methods actually are curried. This is the one
remaining #57 acceptance-criteria item without a ticket of its own.

Acceptance criteria

• Add a shared test helper that asserts a method is curried to a given
  arity — calling it with fewer args returns a function, and every
  partial-application split (f(a)(b), f(a, b)) yields the same result.
• Apply it to every method exported with a curried sibling (the $-prefixed
  / curry* family) across list/, list/utils/, object/, string/,
  boolean/, number/, and function/.
• A method that is documented as curried but fails the assertion is either
  fixed or has its documentation corrected.

Related: #20 (marking curried methods with @curried doc blocks) — the
@curried annotations, once present, give this issue its list of targets.