parser: allow read/write access to symbol table and strictness.

parser: allow read/write access to symbol table and strictness.

Intended usage:

parser reader;
ifstream input_file1, input_file2;
// read the first file...
ex e1 = reader(input_file1);
// ... add extra entry into the symbol table used by parser
symbol x;
parser.get_syms()["x"] = x;
// Disable the parser to introduce new symbols, e.g. to ensure the expression
// in input_file2 contains the same symbols as e1 (read from input_file1).
parser.strict = true;
ex e2;
try {
	e2 = reader(input_file2);
} catch (...) {
	abort();
}