Performance tuning (#1615)

**Performance tuning (#1615)**

Various performance optimizations, without refactoring or messing up too much (hopefully?).

33% - 50% reduction in compilation time for bootstrap.less.

Compilation times for *bootstrap.less*:

| Env.      | Mode                 | 1.5.0-master | 1.5.0-perf-tune | % reduction |
| ------------ | ------------------------ | :----------------: | :-------------------: | :---------------: |
| Node      | lessc (single run) | 719 ms        | 484 ms           | -33%          |
| Node      | JIT'ed (8 runs)   | 509 ms        | 228 ms           | -55%          |
| FF         | Browser             | 2198 ms      | 832 ms           | -62%          |
| Chrome  | Browser             | 1092 ms      | 797 ms           | -27%          |
| IE9        | Browser             | 1720 ms      | 965 ms           | -44%          |