Clean up usage of bison precedence for non-operator keywords.

Clean up usage of bison precedence for non-operator keywords.

Assigning a precedence to a keyword that isn't a kind of expression
operator is rather dangerous, because it might mask grammar
ambiguities that we'd rather know about.  It's much safer to attach
explicit precedences to individual rules, which will affect the
behavior of only that one rule.  Moreover, when we do have to give
a precedence to a non-operator keyword, we should try to give it the
same precedence as IDENT, thereby reducing the risk of surprising
side-effects.

Apply this hard-won knowledge to SET (which I misassigned ages ago
in commit 2647ad658) and some SQL/JSON-related productions
(from commits 6ee30209a, 71bfd1543).

Patch HEAD only, since there's no evidence of actual bugs here.

Discussion: https://postgr.es/m/CADT4RqBPdbsZW7HS1jJP319TMRHs1hzUiP=iRJYR6UqgHCrgNQ@mail.gmail.com