String to Integer (atoi)

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a
challenge, please do not see below and ask yourself what are the
possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no
given input specs). You are responsible to gather all the input
requirements up front.

spoilers alert... click to show requirements for atoi.

Requirements for atoi:
The function first discards as many whitespace characters as necessary
until the first non-whitespace character is found. Then, starting from
this character, takes an optional initial plus or minus sign followed
by as many numerical digits as possible, and interprets them as a
numerical value.

The string can contain additional characters after those that form the
integral number, which are ignored and have no effect on the behavior
of this function.

If the first sequence of non-whitespace characters in str is not a
valid integral number, or if no such sequence exists because either str
is empty or it contains only whitespace characters, no conversion is
performed.

If no valid conversion could be performed, a zero value is returned. If
the correct value is out of the range of representable values, INT_MAX
(2147483647) or INT_MIN (-2147483648) is returned.