reflection no longer uses atPhase and friends

Mentioned methods mutate the global `atPhaseStack` variable, which can
easily lead to imbalances and, ultimately, to the empty stack error.

Luckily for us, there's only one dummy phase, SomePhase, which is used
by runtime reflection, so there is absolutely zero need to invoke atPhase
in non-compiler reflexive universes.
synchronizes PackageScope.negatives

We don't have to serialize access to this cache, but we ought to
make the cache safe for being accessed from multiple threads at once.
synchronizes runtimeMirror
synchronizes toolboxes

We don't guarantee thread-safety of the front end, but everything else
looks good now.
synchronizes symbol materializers

Since runtime reflection doesn't have a luxury of enumerating all classes
on the classpath, it has to materialize symbols for top-level definitions
(packages, classes, objects) on demand.

Someone asks us for a class named `foo.Bar`? Easy. Let's speculatively create
a package named `foo` and then look up `newTypeName("bar")` in its decls.
This lookup, implemented in `SymbolLoaders.PackageScope` tests the waters by
trying to to `Class.forName("foo.Bar")` and then creates a ClassSymbol upon
success (the whole story is a bit longer, but the rest is irrelevant here).

That's all neat, but these non-deterministic mutations of the global symbol
table give a lot of trouble in multi-threaded setting. One of the popular
reflection crashes happens when multiple threads happen to trigger symbol
materialization multiple times for the same symbol, making subsequent
reflective operations stumble upon outrageous stuff like overloaded packages.

Short of significantly changing SymbolLoaders I see no other way than just
to slap a global lock on materialization in runtime reflection.
cleans up initialization of runtime reflection

At first I just tried to remove syntheticCoreClasses from missingHook
and put them into the initializer of freshly created mirrors in order to
reduce the non-determinism in mutations of the global symbol table.

And then it didn't work, crashing on me claiming that AnyRef is missing.
Apparently we still need AnyRefClass in missingHook, just because it's
impossible to initialize (i.e. unpickle) ScalaPackageClass without it.

And then it still didn't work, whining about multiple overloaded defs
of some synthetic symbols. That was really tricky, but I figured it out
as well by initializing ScalaPackageClass first before forcing any
synthetic symbols (see the details in comments).
restores the assertion in missingHook

Now when bootstrapping no longer enters some symbols twice (see the
previous commit for a detailed explanation), we can safely restore
this assertion in missingHook.
runtime reflection: death from thousand threads