update if-let

[primo-ppcg]

Fixes #1189

Code of the following form should not error:

(if-let [[a b] (possibly-nil-or-tuple)] ...)

in accordance with the documentation: If any are false or nil, evaluate the fal form.

Fixes #1191

Macros with side-effects should not be expanded more than once. This is an issue that pre-dates the most recent change.

[primo-ppcg]

The use of symbol? should possibly be replaced with idempotent?, pending the resolution of #1192.

Upon further thought, I believe symbol? is the most correct choice.

[sogaiu]

@primo-ppcg I gave 2a7ea27 a try.

The 20 or so repositories I run tests against are now back to having their tests pass 👍