ExistentialTypeTree.whereClauses are now MemberDefs

Today’s flight back to Lausanne wasn’t as productive as the recent flight
to Minsk (https://github.com/scala/scala/pull/3305), but I noticed
one minor thingie: ExistentialTypeTree had an imprecise type specified
for its whereClauses. This is now fixed.

I didn’t increment PickleFormat.*Version numbers, because this change
introduces only a miniscule incompatibility with what would have been
a meaningless and most likely crash-inducing pickle anyway.
Fix typo in documentation
More robust hyperlink tests for the presentation compiler

Report null symbols in an civilised manner, rather than with a NPE.
Test to show the bug with hyperlinking in macro arguments

Currently, the presentation compiler sees the expansion of macros;
this no longer contains the trees corresponding to the macro arguments,
and hyperlink requests result incorrect results.

https://www.assembla.com/spaces/scala-ide/tickets/1001449#
SI-8064 Automatic position repair for macro expansion

  - Replace NoPosition with the focus of the macro application
  - Focus all range positions, for example, those of spliced arguments
Use macro expandee, rather than expansion, in pres. compiler

The presentation compiler is primarily interested in trees that
represent the code that one sees in the IDE, not the expansion of
macros.

This commit continues to expand macros, but adds a hook in which
the presentation compiler discards the expansion, retaining instead
the expandee. The expandee is attributed with the type of the
expansion, which allows white box macros to work. In addition,
any domain specific errors and warnings issued by the macro will
still be reported, as a side-effect of the expansion.

The failing test from the last commit now correctly resolves
hyperlinks in macro arguments.

Related IDE ticket:

  https://www.assembla.com/spaces/scala-ide/tickets/1001449#

This facility is configured as follows:

    // expand macros as per normal
    -Ymacro-expand:normal

    // don't expand the macro, takes the place of -Ymacro-no-expand
    -Ymacro-expand:none

    // expand macros to compute type and emit warnings,
    // but retain expandee. Set automatically be the presentation
    // compiler
    -Ymacro-expand:discard

This leaves to door ajar for a new option:

    // Don't expand blackbox macros; expand whitebox
    // but retain expandee
    -Ymacro-expand:discard-whitebox-only

The existing test for SI-6812 has been duplicated. One copy exercises
the now-deprecated -Ymacro-no-expand, and the other uses the new
option.
Removes unnecessary generality in the macro engine

In Jan 2013, I submitted a number of pull requests that built up a foundation
for the upcoming type macros pull request. Unfortunately, type macros
ended up being rejected, but the extra generality introduced in advance
still persisted in the compiler until now.

This commit takes care of unused generality in the macro engine,
keeping the internal implementation as well as the public API clean.
Merge pull request #3336 from OlivierBlanvillain/patch-1

Fix typo in documentation
Merge pull request #3332 from xeno-by/topic/existential-type-tree

ExistentialTypeTree.whereClauses are now MemberDefs
Merge pull request #3342 from xeno-by/topic/pres-compiler-macros

Presentation compiler friendliness for macros