gh-153568: Share expression parsing with assignment targets - #157450

The grammar has two versions of almost the same parser: primary for expressions and t_primary for assignment and deletion targets. For factory().field[index], both walk through the call, attribute and subscript. When we try an assignment and then fall back to an expression, that means doing the same work through a different set of rules.

This lets targets use primary too. Parse the expression once, check that its shape is allowed as a target, then copy it into Store or Del context. For factory().field[index] = value, only the outer subscript changes context. The call, attribute and index stay Load, as does the cached expression.

That removes t_primary, star_atom, del_t_atom and the separate target list and tuple rules. It also folds the comparison forwarding rules into their parent, shares the keyword-argument prefix, and gives display items shorter names next to the list, tuple and set rules. There are 15 fewer grammar rules in total.

On my machine, this makes parsing the full standard library and twenty repositories about 9.1% faster overall.

The table compares this change against main at fb46c67d56, using release builds without PGO or LTO. Each result is the median of five paired runs. The timings include tokenization, building the AST and cleanup, without compiling bytecode or running the files.

Part of #153568.

• Issue: Speed up the parser #153568