Rewrite the conversion functions between strings and SQL_NUMERIC_STRUCT.

Rewrite the conversion functions between strings and SQL_NUMERIC_STRUCT.

Aside from making the functions more readable and faster, this fixes
a number of bugs:

1. When converting a large number from decimal to binary, the least
significant byte might be wrong (reported by Walter Couto)

2. In binary->decimal conversion, a number with small precision but large
scale caused a "Floating point exception".

3. In binary->decimal conversion, building the final string was wrong, picking
up "digits" from outside the reserved stack space, if scale was larger than 40

4. The param_string buffer allocated for the result of binary->decimal
conversion was too small.

Also add a test cases for all of these bugs.