Move all the closure related fields in RawFunction into a new class

Move all the closure related fields in RawFunction into a new class
called RawClosureData
Add a field called 'data_' in RawFunction which points to a RawClosureData
object for closure functions, a signature class for signature functions or
an implicit closure function if there is an implicit closure for a regular
function.

This ensures that we don't burn 5 fields in RawFunction for normal functions.
The heap savings from this change on standalone dart is about 67k.

After this change:
New space (0k of 32768k) Old space (886k of 1024k) Code space (0k of 0k)

Prior to this change:
New space (0k of 32768k) Old space (953k of 1024k) Code space (0k of 0k)
Committed: https://code.google.com/p/dart/source/detail?r=12290

[regis, 2012-09-12 21:57:36]

LGTM!