Add numeric for line decorators

This adds support for numeric for loop line decorators. I don't think this is quite right yet, so it may need to take a couple more commits (and then an interactive rebase) before it's actually ready to pull. That is, unless you want to just clean it up yourself.

**Issues**
I think what I added to Statement is correct:
```lua
key"for" * (Name * sym"=" * Ct(Exp * sym"," * Exp * (sym"," * Exp)^-1)) / mark"for" +
```
but the part I'm unsure of is in wrap_decorator:
```lua
stm = {"for", name, {bounds[1][2], bounds[2][2]}, {stm}}
```
I don't think the bounds[][2] is the right way to do this.

The other issue is that make test fails with the last line of the syntax.lua and I'm not entirely sure why.

--Edit--
One other thought which might be better brought up in a new issue: adding numeric for loops for list comprehensions. I'm not totally sure how useful they'd be in real world examples, but I could imagine something like this:
```moonscript
v = [i*2 for i=1,10]
```
Other thoughts, adding when clause to numeric for loops so you could do something similar to this:
```moonscript
v = [i*2 for i=1,10 when i%2==0] -- double evens
```