macro engine refactoring

Macro impl bindings now store more information in signatures.
Previously it was a flattened List[Int] corresponding to flattened paramss,
now it's List[List[Int]] to preserve the lengths of parameter lists.
Also now we distinguish between c.Expr parameters and others.

Previously actual and reference macro signatures were represented as
tuples of vparamss, rets, and sometimes tparams. Now they are all
abstracted behind MacroImplSig.

Finally this patch provides better error messages in cases of
argsc <-> paramsc and argc <-> paramc mismatches.
refactors macro tests:
* condenses some macro-XXX-a/b/c/... bundles
* renames some tests to prepare for type macros
* introduces some additional tests
org.scala-lang.macro-paradise
type macros beta 2

No spec yet, just docs at http://docs.scala-lang.org/overviews/macros/typemacros.html.
List of known issues: https://issues.scala-lang.org/secure/IssueNavigator.jspa?mode=hide&requestId=11912.
SI-6859 type macros no longer crash sbt

By mimicking alias types, macro types no longer trip a sanity check
in sbt 0.12 and 0.13: https://github.com/harrah/xsbt/blob/ef39aeb9c1ea443c17356695758a31152910c4a7/compile/interface/src/main/scala/xsbt/API.scala#L221.

This workaround makes me sad but it's the only option to make type macros
immediately useful. Principled fix pending.
quasiquotes milestone 2

No spec yet, just docs at http://docs.scala-lang.org/overviews/macros/quasiquotes.html.
List of known issues: https://issues.scala-lang.org/secure/IssueNavigator.jspa?mode=hide&requestId=11912.
untyped macros beta 1

No spec yet, just docs at http://docs.scala-lang.org/overviews/macros/untyped-macros.html.
List of known issues: https://issues.scala-lang.org/secure/IssueNavigator.jspa?mode=hide&requestId=11912.