TGE update

I've recently finished up a round of refactors and feature additions to 
Parrot's tree grammar engine (TGE). (TGE is the part of the compiler 
tools that takes a raw parse tree from a language parser written in PGE 
and transforms that parse tree into an abstract syntax tree, then an 
opcode syntax tree, and ultimately into a PIR source file.)

In addition to general cleanups to make the code cleaner, saner, and 
more maintainable, these are the significant feature changes:

* The TGE compiler, the component that compiles tree-grammar source 
files (.tg) down to PIR code, now supports a new, more sensible syntax 
for defining tree-grammar rules:

   transform <rulename> (<pattern>) <modifiers> { <transformation code> }

(The word "transform" is both a verb "to alter something" and a noun "a
rule for making a transformation".)

For example:

   transform result (PAST::Var) :language('PIR') {
     .local pmc result
     result = new 'POST::Var'
     # ...
   }

This defines a transform named 'result' that applies to nodes of type 
'PAST::Var'. In addition, it declares that the syntax used inside the 
body of the rule is PIR. (PIR is the only valid language at the moment, 
but others are on the way...)

* The TGE compiler now handles a 'grammar' keyword, which compiles down 
to the appropriate PIR instructions to create a class that inherits from 
another class:

   grammar MyTreeGrammar is TGE::Grammar;

This means that .tg files now compile down to complete PIR (or bytecode) 
class libraries.

TGE::Grammar is the base class of all tree grammars, but you might 
inherit from your own custom subclass:

   grammar TweenGrammar is TGE::Grammar;

   # ...
   grammar MyTreeGrammar is TweenGrammar;

* The TGE compiler now supports POD, so you can include POD 
documentation in your .tg files.

* Transforms are now methods on the tree-grammar class. You can define 
utility methods or object attributes in the grammar. (For example, the 
TGE::Grammar base class now has an attribute 'symbols' that can be used 
to track symbols during transformations. This is most useful on the 
AST->OST transformation.)

Allison

• TGE update by Allison Randal