syntax: 'var'->'let' 'function'->'func'

- 'var' has some weird scoping connotations with users of JS. better to change it to 'let', which will decide whether to make the variable a local or a global
- 'func' looks visually appealing lol
- some minor refactoring done in cparse.c

examples/variadic.cosmo

@@ -1,5 +1,5 @@
 // adds all args passed (expects numbers)
-function add(start, ...args) 
+func add(start, ...args) 
     // starting at `start`, add up all numbers passed
     local total = start
     for val in args do