Add log and atan2 operations for floats

[nicolas-abril]

HVM exposes log and atan2 operations, but they're not available in Bend except as a hack (using | and & operators).
We should expose them to users with a native operation.

I don't know what syntax to use other than log(a, b) and (log a b), but I don't really like it since it is different from all the other operators.

There multiple way to implement this:

• Parsing log(a, b) as a numeric operation.
• Defining a builtin function log a b = a & b. This is a bit of a hack and I don't love it, but it's the simplest.
• Having a builtin stub log a b that gets converted into the numeric operation as a compiler pass.

[nicolas-abril]

Personally I'm favorable to implementing as option 2 for now, as it is the easiest to change in the future.