OLEAUT32: implement DECIMAL division, (generic) multiplication and (generic) cast to string

[Alex Villaci­s Lasso]

While I prepared the patch for float->DECIMAL conversion (OLEAUT32: 
VarDecFromR[4|8] must use same locale through conversion), I reviewed 
the current status of the DECIMAL math operations. I was surprised (to 
say the least) at how much functionality was missing:

- Cast to string is only implemented for zero-scaled DECIMAL numbers 
smaller than 2^64 - 1; any other scale or value returns E_INVALIDARG.
- No check for NULL pointers as parameters to VarDecMul().
- VarDecMul() only multiplies by integers less than 2^32 -1 (as 
indicated in a FIXME comment)
- VarDecDiv() is entirely unimplemented
- DECIMAL->Single|Double conversions use incorrect scaler for the high 
32 bits of the DECIMAL number
- VarAdd() does not even attempt to handle DECIMAL values, even though 
VarDecAdd() is fully implemented.

All of these issues are handled with the attached patch. I must note 
that the algorithms for generic multiplication and division are based on 
algorithms originally published in _Using_C_, by Lee & Mark Atkinson, 
where they are used to implement multiplication and division for 128-bit 
integers in a C++ example. These algorithms were then modified and 
expanded to handle scaled numbers. In particular, DECIMAL division is 
tricky because no remainder can be returned, so the integer remainder 
has to be scaled and re-divided until it underflows, or exact division 
is achieved. Maybe this is already well-known CS theory on 
floating-point math, but it was an interesting exercise for me. Maybe 
someone with a background on floating-point math can review the division 
for me?

Conformance tests... Coming Soon!

Changelog:
* Implemented generic DECIMAL cast to string
* Implemented generic DECIMAL multiplication
* Implemented DECIMAL division
* Scaling corrected in DECIMAL cast to Single|Double
* VarAdd now handles DECIMAL parameters before trying integer operations.

Alex Villacís Lasso

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