Strawman: a way to rework Scope to make it more flexible.

Parts of this CL will be resubmitted as smaller CLs, but as a whole it is defunct.  Decisions re scopes are documented at https://groups.google.com/group/google-caja-discuss/browse_thread/thread/50db6111ae71c2c9

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We currently have a Scope class which was defined for the Rewriter
scheme.  For each optimization pass I've done, I've ended up writing
my own scope scheme, often on top of Scope.  E.g. the AlphaRenaming
uses an identity map to associate extra info with scopes, and the
array index optimization builds it's own scope tree, so that it can
easily walk from a scope to its children.  The linter uses a scoping
scheme that abstracts away the decision of where declarations are
introduced into scopes and which scopes they are hoisted to, so that
it can make sure that scoping is consistent between JScript and other
interpreters.

I would like to unify these scoping schemes and support more scoping.
Scopes are used in a few different ways:
   # In a rewriter to scope lazily
   # On an entire tree to attach a scope to each node
   # To generate masking errors about violations of Cajita specific
     invariants (such as forbidding declaration of the name "cajita")
   # To identify the kind of symbol -- function declaration, local
     function name, exception, data, globals
   # To collect information about declarations
   # To resolve references to declaration points or to scopes
   # To detect redelcaration
   # To emulate ES5 scoping rules
   # To emulate broken IE scoping
   # To collect uses of a variable
   # To determine whether/where a symbol is used as a LeftHandSideExpression
   # To collect uses in narrower scopes.
   # To collect the set of symbols from wider scopes.

There are a few separable concerns here:
   # Applying scoping rules of a given variant of JS -- exception
     hoisting, whether function constructors' names intrude on the
     containing scope to introduce scopes, and collect declarations
   # Associating scopes with parse tree nodes Collecting usage
     information

Please see ScopeListener for a general interface to collecting and
collating Scope information.  See ScopeAnalyzer for a general
implementation that implements a scoping scheme, and its concrete
implementations: ES5ScopeAnalyzer, JScriptScopeAnalyzer,
WorstCaseScopeAnalyzer.

The ScopeAnalyzerTests give an idea of how ScopeAnalyzer works.

Finally, Scope has been rewritten to use ES5ScopeAnalyzer.

[MikeSamuel, 2009-10-26 22:54:16]

Commented ScopeAnalyzer and cleaned up confusion between "definition" and
"declaration."  See the glossary in ScopeAnalyzer's class comment for details.