Adds support for var statements at the global level.
Adds:
- analyzer/Variables: keeps track of the variables in scope (either local or global).
- ast/Identifier: AST node for an identifier.
- The optimizer will replace constant identifiers with their constant value (in a later commit).
- The semantic analyzer will set the variable field for local or global variable identifiers.
- ast/VarStatement: AST node for a var statement.
- A single var statement will generate one VarStatement per variable declared.
- model/Variable: Describes a variable, including its index within its scope.
