Fixed some decimal implementation errata.

Some spelling errors and redundant includes are fixed.
Fix a link error for IntelDFP as shared Library

The Intel DFP library, when compiled in C++, doesn't expose symbols
declared in 'extern "C"' blocks which are also const.   By making
these constants have an explicit 'extern "C"' qualifier on the
declaration, rather than in a block seems  to resolve the issue.
Add BID conversion functions to DecimalImplUtil.

The BID format is now the default format for 'bdldfp', so it makes
sense to have "convert to BID" functionality in the library.
Support exists, presently, only for IntelDFP.  A portable 'DPD'
conversion function will be added later.
Added BID conversion functions to ConvertUtil.

The BinaryIntegralDecimal format conversion functions were added
to DecimalConvertUtil.  These functions will be useful for protocols
which use 'BID' format on the wire, rather than 'DPD'.
Merge branch 'decimal-conversion-work' into decimal-omnibus-merge
Add decimal encode and decode functions

Add functions 'decimal64ToMultiWidthEncoding',
'decimal64FromMultiWidthEncoding', 'decimal64ToVariableWidthEncoding',
and 'decimal64FromVariableWidthEncoding' to 'bdldfp_decimalconvertutil'.
Change to use BID for 8-byte squish.
Update shifted integer constants to be unsigned

Shifting signed integer constants into the sign bit is technically
undefined behavior. Per Mike G's suggestion, we changed the constants to
be unsigned.
[APPROVAL] review/decimal-encode-drqs-42817882 to master