perf(*): Small performance changes for one-time binding interpolation

Add a `clone` function to the interpolate function. Use `clone` when interpolating text

Do not add multiple times the same post digest function on one-time binding


The benchmark was done using http://plnkr.co/edit/rJOqa0oh35Y0xBhTYZ6w
Without this patch, the initial digest is ~40% slower for expressions using one-time binding. With this patch, the initial digest is ~20% slower