SI-8192 adds ClassSymbol.isPrimaryConstructor

Exposes a popular code pattern in macros as a dedicated reflection API.
This simple commit, however, ended up being not so simple, as it often
happens with our compiler.

When writing a test for the new API, I realized that our (pre-existing)
MethodSymbol.isPrimaryConstructor API returns nonsensical results for
implementation artifacts (trait mixin ctors, module class ctors).

What’s even more funny is that according to our reflection internals,
even Java classes have primary constructors. Well, that’s not surprising,
because `primaryConstructor` is just `decl(ctorName).alternatives.head`.
Good thing that package classes don’t have constructors or that would
elevate the situation to three fries short of a happy meal.

At the moment, I’m too scared to fiddle with internal#Symbol.primaryConstructor,
because that could easily break someone right before RC1, so I simply
documented the findings in SI-8193 and postponed the actual work, except
for one thing - isJavaDefined symbols no longer have primary constructors.
SI-6484 adds Universe.typeOf[T](x: T)

This is a nice addition to Universe.typeOf[T], quite frequently useful
for inspecting ad-hoc types.
SI-8194 adds Universe.symbolOf[T]

A very frequent use for a result of typeOf is obtaining an underlying
type symbol. Another thing that comes up occasionally at stack overflow
is a request to add facilities for reification of symbols.

This naturally suggests that our reflection API would benefit from a method
called symbolOf that can take a term or type argument and return an underlying
symbol.

While an API to extract a term symbol from an expression needs some time
to be designed and then implemented robustly (we don’t have untyped macros
so we’ll have to account for various desugarings), meaning that we probably
won’t have time for that in 2.11, a type symbol extractor seems to be
a very low-hanging fruit.
adds showDeclaration(sym: Symbol): String

As per Paul’s request, this commit exposes Symbol.defString, although
in a different way to ensure consistency with our other prettyprinting
facilities provided in the reflection API.