User-friendly syntax-error messages for any Menhir grammar

stele writes the syntax-error messages of a Menhir grammar at build time. From menhir --list-errors and the grammar's .cmly, it produces a message for every error state: what could come next, which opening delimiter is still open, and, where the grammar uses %on_error_reduce, which construct the parser assumed complete. Each message is checked against the parser's tables, and the messages can be pinned in golden files, so a grammar change shows up as a diff to review. An optional config file improves the wording, an overrides file holds hand-written messages, and stele tune suggests changes to the %on_error_reduce annotations. The stele.runtime library, which depends only on menhirLib, turns the location markers in a message into source spans when the parser fails.