The existing grammar was causing simple expressions to be incorrectly
parsed as a subtype indication. This change raises the precedence for
a discrete choice expression to address that issue.
Updated the test corpus to reflect the parsing correction. Additional
test cases were added for discrete choice.
Instead, package_declaration replaces it. This provides
better folding in nvim: otherwise, the final ';' was not
part of the tree node, and thus the last line "end P;" was
not folded.
