hooks for typecheck and expansion of macro defs

Creates MacroPlugin, a sister interface of AnalyzerPlugin in the namer/typer
extensibility interface.

Exposes `pluginsTypedMacroBody`, `pluginsMacroExpand`, `pluginsMacroArgs`
and `pluginsMacroRuntime` in the macro plugin interface. This will
make it easy to prototype changes to the macro engine without disturbing scala/scala.
humane reporting of macro impl binding version errors

Macro defs are linked to macro impls by the virtue of MacroImplBinding
structures that are persisted between compilation runs serialized within
instances of macroImpl annotations.

Along with the evolution of our macro engine, we sometimes have to evolve
the format of MacroImplBinding, which means that it has to be versioned.
Version mismatches are checked upon every macro expansion, ensuring that
macros that we expand were compiled with exactly the same version of the
macro engine that we’re running.

That’s all really cool apart from the fact that version mismatches result
in aborting the entire compilation with an obscure message without giving
a hint about the culprits.

This commit improves the situation by providing pretty per-expansion
compilation errors that tell the programmer what macro expansions are
at fault and what macro engines were used to compile them.
manifests that Namers.mkTypeCompleter is flag-agnostic

When introducing `FlagAgnosticCompleter` and `FlagAssigningCompleter`,
I have overlooked the completers created in namers. These completers
only assign type signatures, not flags, because they are only created
for symbols originating from sources and such symbols have their flags
assigned based on modifiers and source def trees when they are entered.
unprivates important helpers in Namers.scala

This is the first of two commits that enable hooks necessary to implement
macro annotations in an honest, hackless compiler plugin.

This particular commit turns certain helpers into public methods. Of course,
there is a probability that with the evolution of macro paradise, I will need
more helper methods, and those will have to be called via reflection, but
at least for now it's nice to have a reflection-less plugin :)
hooks for naming and synthesis in Namers.scala and Typers.scala

Interestingly enough, despite of the implementation surface being
quite noticeable, it is enough to hijack just `enterSym` and typechecking
of stats for packages, templates and blocks in order to enable macro annotations.
That and `ensureCompanionObject`, which I couldn't abstract away so far.

An architectural note: given that a hooked method is called `X`,
there are two implementations of this method. `pluginsX` is defined in
AnalyzerPlugins.scala and lets macro plugins customize `X`.
`standardX` is defined next to `X` and provides a default implementation.
Finally `X` is changed to trivially forward to `pluginsX`.

Existing and future callers of `X` now can be completely oblivious of the
introduced hooks, because calls to `X` will continue working and will be
correctly hooked. This makes the infrastructure more robust.

The only downside is that in case when a macro plugin wants to call
into the default implementation, it needs to call `standardX`, because
`X` will lead to a stack overflow. However, in my opinion this not a big
problem, because such failures are load and clear + for every `pluginsX`
we actually provide documentation that says what is its standard impl is.