Add name-scanning pass to compiler

Compile::discover_locals now takes a Vec<SpExpr> instead of a single
SpExpr. It is also called before the compiler starts building the list
of instructions so that all names in the scope are available at
compile-time.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/factorial.sy

@@ -1,8 +1,5 @@
 [ :x x x ] :dup
 
-# This a dumb little workaround to allow for recursion so factorial inside of
-# the function definition will look in the correct scope
-0 :factorial
 [
     dup!
     [dup! 1 -! factorial! *!]