Upgrade to ANTLR4

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Revert "Upgrade to ANTLR4"

This reverts commit d897f9375ff3abcec0a201b53823f97ee2848e89. It's not a
matter of simply bumping up the version number in Maven POM.
Lexer: Make VerbatimStringChar a fragment

VerbatimStringChar is not a token so turn it into a fragment.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: First take types, expressions, and patterns

Most of the grammar is disabled but it is good enough for expressions
such as:

  let x = "hello"

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: Remove duplicate tokens

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: Types

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: Fix type grammar rule

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Parser: Expression improvements

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Lexer: Fix Int32

Signed-off-by: Pekka Enberg <penberg@kernel.org>
AST: Constants

Signed-off-by: Pekka Enberg <penberg@kernel.org>