Improve map performance by using hash tables
Don't re-evaluation unnecessarily
Merge pull request #563 from xzyfer/wip/fast-maps

Give maps a speed boost
Fixes memory leaks with C functions and Sass_Values

Sass_Values that are created programmatic may allocate
memory on the heap. Since we may return this Sass_Value
back to libsass, the implementer cannot take care of
freeing this memory. Therefore libsass should free
Sass_Values that get returned by c_functions.
Changes usage of new to malloc for better C compatibility
Updates sass2scss to latest version