formulet assertion should count bindings, not binding-pairs

[jjttjj]

This fixes a small bug where you couldn't use formulet with a non-even number of bindings.

(macroexpand '(formulet [a (cell 123)]
                           (+ 1 2)))
Unexpected error (AssertionError) macroexpanding formulet at (REPL:1:1).
Assert failed: first argument must be a vector of binding pairs

[daveyarwood]

I also submitted a PR over 2 years ago with this same fix: #38

I'd love to see this fixed and released :)

[alandipert]

Fixed with #38