Add some bitvector rules

This moves #31 forward!

spec/changelog.tex

@@ -14,10 +14,12 @@ Breaking changes:
 \end{itemize}
 
 \noindent
-Clarifications:
+Clarifications and corrected errors:
 \begin{itemize}
   \item Clarify that the \texttt{:args} annotation in \texttt{anchor} can be
         omitted if the list is empty.
+  \item Fix mistake in proof grammar.  It now uses the \texttt{context\_annotation}
+        non-terminal in the rule for the \texttt{anchor} command.
 \end{itemize}
 
 \subsection*{0.3 --- \DTMdisplaydate{2023}{02}{10}{-1}}