This reduces the number of allocations Miri makes drastically. The
`const_to_ptr` function was a lame hack that allocated for every since simple
constant, and all of those are avoided now, except for one extra allocation each
for string and bytestring literals which will be fixed in a followup commit.
There are a number of hacks such as `eval_operand_to_ptr` left over from this
commit, which will also be fixed in followup commits.
