Add hashing of public names defined in a source file.

A hash for given name in a source file is computed by combining
hashes of all definitions with given name. When hashing a single
definition we take into account all information about it except nested
definitions. For example, if we have following definition

class Foo[T] {
  def bar(x: Int): Int = ???
}

hash sum for `Foo` will include the fact that we have a class with
a single type parameter but it won't include hash sum of `bar` method.

Computed hash sums are location-sensitive. Each definition is hashed along
with its location so we properly detect cases when definition's signature
stays the same but it's moved around in the same compilation unit.

The location is defined as sequence of selections. Each selection consists
of a name and name type. The name type is either term name or type name.
Scala specification (9.2) guarantees that each publicly visible definition
is uniquely identified by a sequence of such selectors.

For example, if we have:

object Foo {
  class Bar { def abc: Int }
}

then location of `abc` is Seq((TermName, Foo), (TypeName, Bar))

It's worth mentioning that we track name-hash pairs separately for
regular (non implicit) and implicit members. That's required for name
hashing algorithm because it does not apply its heuristic when implicit
members are being modified.

Another important characteristic is that we include all inherited members
when computing name hashes.

Here comes the detailed list of changes made in this commit:

  * HashAPI has new parameter `includeDefinitions` that allows
    shallow hashing of Structures (where we do not compute hashes
    recursively)
  * HashAPI exposes `finalizeHash` method that allow one to capture
    current hash at any time. This is useful if you want to hash a list of
    definitions and not just whole `SourceAPI`.
  * NameHashing implements actual extraction of public definitions,
    grouping them by simple name and computing hash sums for each group
    using HashAPI
  * `Source` class (defined in interface/other file) has been extended to
    include `_internalOnly_nameHashes` field. This field stores
    NameHashes data structure for given source file. The NameHashes
    stores two separate collections of name-hash pairs for regular and
    implicit members.
    The prefix `_internalOnly_` is used to indicate that this is not an
    official incremental compiler's or sbt's API and it's for use by
    incremental compiler internals only. We had to use such a prefix
    because the `datatype` code generator doesn't support emitting access
    modifiers
  * `AnalysisCallback` implementation has been modified to gather all
    name hashes and store them in the Source object
  * TestCaseGenerators has been modified to implement generation of
    NameHashes
  * The NameHashingSpecification contains a few unit tests that make sure
    that the basic functionality works properly
Add overloaded constructor to HashAPI for backwards compatibility.

Add a variant of constructor to `HashAPI` that is binary and source
backwards compatible with sbt 0.13.0.
Get rid of MurmurHash.finalizeHash import in HashAPI.

That import is shadowed by the local definition of `finalizeHash`
introduced to HashAPI class.
Access `Relations.names` conditionally in TextAnalysisFormat

The `Relations.name` is available only when name hashing flag is enabled.

This fixes a regression introduced in
304796bb7aa10223e06527a1c0a183f6689c82a8.
Push down `toString` implementation in Relations hierarchy.

Each of Relations implementation should have specific `toString`
implementation. For example, only `MRelationsNameHashing` implementation
should be printing used names in toString representation.
added sbt-mode to Community-Plugins
Merge remote-tracking branch 'gkossakowski/compute-name-hashes' into 0.13