`arguments.length` in a typed function folds to a constant. In a function
declaration it was the declared parameter count even when the function is
called through a value with more arguments: Node prints 4 for
`[1].map(f)` with `function f(v) { return v + arguments.length; }`, the
compiled program printed 2.
A top-level function declaration that reads `arguments` and that the
program uses as a value now gets a count slot. Its value is an adapter
that stores the parameter count of the function type the value is passed
as (3 for map, 4 for reduce, 2 for a sort comparator) and forwards; the
function reads the slot on entry and clears it, so a direct call sees the
declared count.
Function expressions and declarations passed where the expected function
type has optional or rest parameters (a replace callback) are called with
a varying count and are refused with SC1090 instead of folding to a wrong
count, as are methods, nested functions, and arrows inside a function
used as a value.
