common/json_parse_simple: make convenience functions inline

[whitslack]

json_tok_streq(…) and json_get_member(…) are convenience wrappers for json_tok_strneq(…) and json_get_membern(…) respectively. Unfortunately, using them incurs a performance penalty in the common case where they are called with a string literal argument because the compiler is unable to substitute a compile-time constant in place of the buried call to strlen(…).

For example,

json_get_member(buf, tok, "example");

…will have worse performance than…

json_get_membern(buf, tok, "example", strlen("example"));

…because the former is forced to scan over "example" at run-time to count its length whereas the latter is able to elide the strlen(…) call at compile time.

Hoist these convenience functions up into common/json_parse_simple.h and mark them as inline so that the compiler can elide the strlen(…) call in the common case of calling these functions with a string literal argument.

Checklist

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