updated grammar comment

doc only, no code changes
rename AST structure for expressions from Abstract to Expr
rename function Parser.parse to Parser.parseExpr

top-level parser will actually parse /declarations/ not /expressions/
created new AST structure, expressions, declarations, types as substructures
brought Pattern structure under AST
(wip) parser for type expressions
(wip) refactor parser for type exprs to return pair of type AST and unconsumed tokens
removed nested structure Parser.Type, replaced with local
(wip) parser for declarations
clean up some show boilerplate
clean up some more show boilerplate
refactored monomorphic AST, extended with Type and Decl
moved debug flag to top of Parser module
begin (re-)adding tests for parser
test coverage of most expressions, as part of val bindings

enable less pain refactoring parser to have parseExpr fn