stringToInt feature implemented

[IvanLeu]

Function strinToInt that returns int64_t is implemented using efficient algorithm of converting string to int. For the sake of initial string data safety I used helper c-style string(But it can be implemented without any helper strings by traversing string data field). Also the sign of the number is considered. Error handling is implemented using this frameworks error handling tools. Also new error code was added for better debugging. As helper function was added function stringConvertToCharArr that converts TString type to c-style string type.

[Tnirpps]

this is a kind of duplication of the previous function, because int can always be implicitly converted to int64_t

[Tnirpps]

Suggested change

	if(res == NULL) {
	setError(ERR_NULL_POINTER);
	}
	if (isError()) {
	return res;
	}
	if (res == NULL) {
	setError(ERR_NULL_POINTER);
	return NULL;
	}

[Tnirpps]

1. Segfault when len = 0
2. Please, check code style convention
3. It is not necessary to allocate a C-string, better use s.data and s.size

[Tnirpps]

See this how to prevent signed overflow
Now it is UB

[Tnirpps]

Everything is good but this still can cause overflow val * 10. It'l be better to check smth like this:

val > (INT64_MAX - digit) / 10

be careful with rounding, it requires divisibility check

[Tnirpps]

If function can change error state, it have to clear error in the begin, to prevent smth like that:

// err = no_err
foo()                              // set err to smth, but result wasn't checked
...
//err = some_err
bar()                              // no error actually happened but flag wasn't cleaned
if (err == some_err) {             // check error in bar() result
    ...                            // but goes here because err after foo()
}

[Tnirpps]

although it is better to always check the error after each function call

[Tnirpps]

Such a nice code!
just change to >= and add test for stringToInt(INT64_MAX)

[Tnirpps]

it will probably overflow with INT64_MIN due to the asymmetric range but you may just leave TODO

[Tnirpps]

Nice