Make bv{add,mul,and,or,xor,xnor} left-associative (#2955)

Make bv{add,mul,and,or,xor,xnor} left-associative (#2955)

The most recent version of SMT-LIB defines bv{add,mul,and,or,xor,xnor}
[0, 1] as left-associative. CVC4 treats all but bvxnor as having
variable arity anyway but the arity check was too strict when using
`--strict-parsing`.  This commit changes the strict parsing check. For
bvxnor, it adds code to the parser that expands an application of bvxnor
into multiple applications of a binary bvxnor if needed.

References:
[0] http://smtlib.cs.uiowa.edu/theories-FixedSizeBitVectors.shtml (bvand,
bvor, bvadd, bvmul)
[1] http://smtlib.cs.uiowa.edu/logics-all.shtml#QF_BV (bvxor, bvxnor)