[Scons-dev] Vector to speed up SCons through DefaultEnvironment memoization

[anatoly techtonik]

When you create Environment only with specific tools,
DefaultEnvironment is still called and queries all tools.

The known workaround is to explicitly initialize
DefaultEnvironment with subset of tools. But explicit
workaround is ugly, so it should be possible to pass
tools argument from Environment to DefaultEnvironment
and memoize (cache) DefaultEnvironment for this
specific combination of tools.

I think this will save us more than 20% of time on tests.
-- 
anatoly t.