Add la_mult_pos and la_mult_neg rules

This PR adds two rules la_mult_pos and la_mult_neg to the standard.

spec/changelog.tex

@@ -5,6 +5,7 @@ Proof rules:
   \item Addition of a section describing bitvector proofs.
   \item Bitblasting rules: \proofRule{bitblast_extract},  \proofRule{bitblast_add},
       \proofRule{bitblast_ult}.
+  \item Addition of rules \proofRule{la_mult_pos} and \proofRule{la_mult_neg} to describe multiplication with a positive or negative factor.
 \end{itemize}
 
 \noindent