Deprecate Valid() and add Wellformed() to replace it

[fxamacker]

RFC 8949 distinctly defines what is "Valid" and what is "Well-formed".

Although the old docs for Valid() correctly said it checks for well-formedness, Valid() is not an appropriate function name because it can lead to confusion.

Proposed Solution

• Add Wellformed() as a replacement for Valid().
• For now, deprecate Valid() and make it call Wellformed(). This makes Valid() behave the same in v2.5.0 as prior releases.

Wellformed() checks whether data is a well-formed encoded CBOR data item and that it complies with additional restrictions such as MaxNestedLevels, MaxArrayElements, MaxMapPairs, etc. If there are any remaining bytes after the encoded CBOR data item, then ExtraneousDataError is returned.

⚠️ Projects should begin replacing calls to Valid() with calls to WellFormed().