Optimization: use "const ref" for function arguments

[alastairreid]

When generating C++ code, using const ref is faster for arrays, records and large ints/bitvectors. (But it is probably slower for values that already fit in registers.)

Note that applying the same optimization for C code is, in principle, easy: "just add a star in front of the variable's name" but rapidly becomes tedious and error prone as we have to insert "&" in front of function arguments, convert function arguments to l-values, (optionally?) optimize "&*" when a function argument is passed further down the call graph and do the right thing in the foreign function interface.
So we will probably not try to optimize generated C code in this way.