Allow binding of function return values

[mkremins]

Currently you can call all of these functions within a sifting pattern, but you can't bind a function's return value to a logic variable. In practice, this means that only some of the functions (those returning a boolean or usefully boolean-coercible value) are useful, and functions can only be used to directly check yes/no conditions.

The planned syntax for binding function return values looks like this:

(event ?e1 where
  ?prevEntityID: (dec ?e1))

...i.e., an unbound undotted logic variable followed directly by a function call.

The relevant TODO comment in the current version of compiler.js is located here:

winnow/compiler.js

Line 170 in 837807e

// 3. lhs is unbound undotted lvar, rhs is fncall (TODO handle this case)