compress numerical expressions (#1513)

safe operations
- `a === b` => `a == b`
- `a + -b`  => `a - b`
- `-a + b`  => `b - a`
- `a+ +b`   => `+b+a`

associative operations
(bit-wise operations are safe, otherwise `unsafe_math`)
- `a + (b + c)`       => `(a + b) + c`
- `(n + 2) + 3`       => `5 + n`
- `(2 * n) * 3`       => `6 * n`
- `(a | 1) | (2 | d)` => `(3 | a) | b`

fixes #412

README.md

@@ -350,6 +350,9 @@ to set `true`; it's effectively a shortcut for `foo=true`).
   comparison are switching. Compression only works if both `comparisons` and
   `unsafe_comps` are both set to true.
 
+- `unsafe_math` (default: false) -- optimize numerical expressions like
+  `2 * x * 3` into `6 * x`, which may give imprecise floating point results.
+
 - `unsafe_proto` (default: false) -- optimize expressions like
   `Array.prototype.slice.call(a)` into `[].slice.call(a)`