refactors importers

Introduces better names, factors out recreation of symbols, trees, types
and completes into separate methods, so that they can be overridden in
specialized importers.

The original motivation for this refactoring was to support JIT
compilation of macros, but I think that most of the introduced
improvements to code quality will be useful in trunk.
refactors macro compilation

Upgrades the way that macro defs are compiled by factoring out most of
the logic in typedMacroBody and related errors in ContextErrors into an
standalone cake. This leads to tighter cohesion and better code reuse
as the cake is isolated from the rest of the compiler and is much easier
to evolve than just a method body.

Increased convenience of coding macro compilation allowed me to further
clarify the implementation of the macro engine (e.g. take a look at
Validators.scala) and to easily implement additional features, namely:

1) Parameters and return type of macro implementations can now be plain
c.Tree's instead of previously mandatory c.Expr's. This makes macros more
lightweight as there are a lot of situations when one doesn't need to
splice macro params (the only motivation to use exprs over trees). Also
as we're on the verge of having quasiquotes in trunk, there soon will be
no reason to use exprs at all, since quasiquotes can splice everything.

2) Macro implementations can now be defined in bundles, standalone cakes
built around a macro context: http://docs.scala-lang.org/overviews/macros/bundles.html.
This further reduces boilerplate by simplifying implementations complex
macros due to the fact that macro programmers no longer need to play
path-dependent games to use helpers.
refactors macro runtimes

Following typedMacroBody, macroRuntime along with its friends has also
been moved out into a separate component.
changes some definitions to use Symbol.map
applying Jason's aesthetics suggestion
unifies handling of T's in various analyses of Foo[T]'s