Imports referenced more than once seem to be parsed multiple times

Hi guys,

I just notice the build for a somehow complex less file was taking about 4s. I did some debug and found that the imports are parsed several times. Even when they were already previously parsed.

I have made a simple change to the import-manager as a proof of concept

https://github.com/royriojas/less.js/commit/b84e7c6a51f3f6221eb31ac3e1639caaa7c1c57f

with that small change the build time was reduced to 1.5s, I guess because in my use case some files: (constants, mixins and others) were included several times from different files

The change I did is very naive, and we can make sure we create a cache for each instance of the less object created.

Also, if we store this cache to a local file in the users filesystem we can reuse it for the next run and this will a build be reduced to just milliseconds. Are there any plans of enabling something like that. Would be really nice to have.

Start with the import-manager and the proof-of-concept commit linked in the issue, then reproduce the reported case of a complex Less file with repeated imports. Compare parsing counts and build times before and after; done means repeated imports are not parsed unnecessarily and the existing behavior remains correct.