Planned code cleanups

[alastairreid]

ASLi is a work in progress. We are actively extending it, improving testing and cleaning up the code.
Some of the planned cleanups are

• Combine the representations of function return ('Stmt_FunReturn') and procedure return ('Stmt_ProcReturn') into a single node
• Drop the _Cond suffix from s_elsif and e_elsif constructors (maybe do a general cleanup of the whole AST naming scheme?)
• Consider changing 'S_If' to 'S_if (s_elsif list) stmts' - treating the initial if more uniformly
• Combine the representations of local variable declarations ('Stmt_VarDecl', 'Stmt_ConstDecl', 'Stmt_VarDeclsNoInit') into a single node
• Consider combining the representation of configuration variables ('Decl_Config'), global variables ('Decl_Var') and global constants ('Decl_Const')
• Consider merging the representation of tuples and records