String to Integer (atoi)

An old form reader turns whatever people typed into a 32-bit integer. It never rejects input: it reads as much of a number as it can find at the start and ignores the rest.

Write string_to_int(s: str) -> int that follows these rules, in order:

1. Skip leading space characters ' '. Only spaces are skipped; any other character ends this step.
2. If the next character is '+' or '-', read it as the sign. At most one sign is allowed. With no sign the number is positive.
3. Read digits 0-9 until the first character that isn't a digit, or the end of the string. Leading zeros are fine. Everything after the digits is ignored.
4. If step 3 read no digits at all, the answer is 0.
5. If the number is outside the 32-bit range [-2^31, 2^31 - 1], clamp it: anything below -2147483648 becomes -2147483648, and anything above 2147483647 becomes 2147483647.

string_to_int("  -0042 apples")     # -42  (spaces, sign, digits; " apples" is ignored)
string_to_int("+17")                # 17
string_to_int("3.99")               # 3    (the "." stops the digits)
string_to_int("about 12")           # 0    ("a" is not a space, sign or digit)
string_to_int("-+5")                # 0    (a second sign is not a digit)
string_to_int("  - 5")              # 0    (no digits right after the sign)
string_to_int("98765432109")        # 2147483647   (too big, clamped)
string_to_int("-2147483649")        # -2147483648  (too small, clamped)

Constraints: 0 <= len(s) <= 200; s holds English letters, digits, spaces, '+', '-' and '.'.

In C++ and Java, the digits can run far past what even a 64-bit integer holds, so stop growing the number once it is out of range.