Clean up bqn-symbols-doc.el

* bqn-symbols-doc.el (subr-x): Remove dependency.
(bqn-symbols-doc--symbol-doc-table): Have the hash table generated at
compile time instead of writing out the printed representation.
(bqn-symbols-doc--symbols): Replace 'hash-table-keys' with 'maphash'.
(bqn-symbols-doc--get-doc): Replace legacy 'when-let' with 'and'.