Clash with reserved word in AbstractBuf leads to SMT2 error

[msooseth]

Example to reproduce is to go into cabal repl test and then:

a = ReadWord (Lit 0x18) (AbstractBuf "as")
b = ReadWord (Lit 0x32) (AbstractBuf "bimm")
checkEquiv a b
Error "Unable to parse solver output: (error \"line 225 column 16: invalid constant declaration, symbol expected\")"

Notice the AbstractBuf "as" which is what triggers the error. What happens in the SMT2 is that we have a buffer declared as:

(declare-const as (Array (_ BitVec 256) (_ BitVec 8)))

But as is a reserved word in SMT2. This could happen with others, e.g. buffer, or select etc. We likely want to have them numbered. I'll try to fix this and see how it goes :)

Not a huge blocker, just a minor bug. It was found by quickcheck, actually. One of the builds failed and I was curious why.