[SMTChecker] Use natspec annotation to abstract function

Some contracts use functions like `pow` or `sqrt`, which are pretty hard to verify.
We could use natspec annotations to allow the user to force calls to that function to be abstracted as UF, which might help in some cases:
```
// @ spec-smtchecker abstract
function pow(uint a, uint b) {
...heavy stuff...
}
```

Start by reading the SMTChecker implementation and the existing NatSpec annotation handling. Trace how function calls are represented for verification and determine where the proposed `spec-smtchecker abstract` annotation would be recognized. Done means calls to annotated functions are abstracted as uninterpreted functions without changing unannotated behavior, with coverage for cases such as `pow` or `sqrt`.

Written by the indexing model from the issue text.