previously, when attempting to locate a mixin/fn/variable, we would
check if it existing simply by calling `get_{mixin,fn,var}`. this caused
*large* performance regressions in bootstrap, where almost 15% of the
execution time was spent in these functions. the cause for this was that
`get_*` both allocates a new String and boxes the error.
There existed issues related to multithreaded tests that are difficult
to resolve. In the future interning may be reimplemented but in a more
limited capacity.
The motivation behind interning *values* was that it appeared checking
for named colors was responsible for much of the time lost when parsing
unquoted strings. If interning were to be reimplemented, it may make
sense to limit it solely to identifiers and style properties.
This reverts commit 27a9578b17a8ed8227cb190af38762111edae58f.
This ended up not being the desired behavior. It seems that instead of
checking the global scope first, variables declared with `!global` are
inserted into both the local and global scopes.
