Limit use of sscanf only to double parsing, using a custom implementa…

…tion for {u}int{32|64} parsing. (Azure#974)

* Limit use of sscanf only to double parsing, using a custom implementation
for {u}int{32|64} parsing.

* Add a few more atod test cases.

* Add explicit casts to avoid implicit conversion warnings.

* Fix bug where input is too small and add test cases.

* Address PR feedback - add and update comments.