Add math support to embedded interpolation language

[mitchellh]

Fixes #686

This adds basic arithmetic to the embedded interpolation language: add, subtract, multiply, divide, modulo for both ints and floats (except mod for floats, of course). This lets you do fancy things like ${count.index+1}. The changes made in this PR:

• Add math operations to lexer (+, -, *, /, %)
• Parse math in parser: expr OP expr and '(' expr ')' (for orders of operations).
• Arithmetic operation node into the AST
• Type check for arithmetic
• AST transformation: replace arithmetic operations with function calls to builtins to do the actual math. This lets us do it cleanly in Go vs. creating a special case evaluable node.
• Added a few implicit conversions to make this more usable: int to float, float to int, and float to string.

[mitchellh]

Ready for review as best as possible @phinze. I don't expect you'll have to see this area of the code much, but could be useful to see how something like this is added. :)

[phinze]

"You don't get fload mod, because if you need float mod in your terraform config, you're doing something very wrong."

^^ This the logic? Sounds about right to me. 😏

[mitchellh]

Yeah, float modulus doesn't make sense or even work (syntax error in Go).

[phinze]

Well there is http://golang.org/pkg/math/#Mod but the logic still stands

[phinze]

LGTM!

Obligatory:

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