optimizes Scala reflection GIL

First of all, GIL should only apply to runtime reflection, because noone
is going to run toolboxes in multiple threads: a) that's impossible, b/c
the compiler isn't thread safe, b) ToolBox api prevents that.

Secondly, the only things in symbols which require synchronization are:
1) info/validTo (completers aren't thread-safe),
2) rawInfo and its dependencies (it shares a mutable field with info)
3) non-trivial caches like in typeAsMemberOfLock

If you think about it, other things like sourceModule or associatedFile
don't need synchronization, because they are either set up when a symbol
is created or cloned or when it's completed.

We can say that symbols can be in four possible states: 1) being created,
2) created, but not yet initialized, 3) initializing, 4) initialized.
#1 doesn't need synchronization, because it's always performed by a
single thread. #2 and #4 don't need synchronization either, because the
only mutation symbols in runtime reflection can undergo is init. #3 is
dangerous and needs protection.