Previously, we would use lossy strtod() conversion. This was bad,
especially since we switched from internally storing Calculator
state in a double to storing it in the KeypadValue class
some time ago. This commit adds a constructor for the KeypadValue
class that is not lossy by using strtoll(). It handles numbers
with and without decimal points as well as negative numbers
correctly.
This commit adds a basic KeypadValue class which abstracts away
Keypad's internal representation in a slightly simpler format.
This will allow arithmetic operations in the Calculator to not
lose any precision. In cases where losing precision is necessary,
an explicit conversion operator to double is provided, as well as
an explicit constructor from double.
