z3_solver: unary operator parse support

This solve the following issue reported by wexlermi

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Crest-Z3 does not allow for the unary minus operator
This fork of Crest does not allow for the unary minus operator. For instance, the following constraint will crash Crest:

if (x*x*x == -8) {
printf("here");
}

Subtraction seems to be allowed, but any time the unary minus operator appears, Crest crashes.
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size_t -> int
Update symbolic_expression.cc

The location of whitespace is important, it should be set appropriately to meet the format used in the function Z3Solver::ParseStatement(...)!
Update symbolic_expression.cc

'Segment Fault' arises when '\n' is the delimiting character. Note that the delimiting character ('\n') is discarded and not written to buf when the function getline is called at line 79. And a null character ('\0') is automatically appended to buf. So I think the delimiting character should be '\0'
Merge pull request #2 from dqx/patch-1

Update symbolic_expression.cc
Merge branch 'patch-2' of https://github.com/dqx/crest-z3
Merge branch 'patch-1' of https://github.com/dqx/crest-z3
Merge branch 'master' of github.com:heechul/crest-z3