SWIFT-822 Prefix remaining BSON types

[kmahar]

Leftovers from the last PR. Adds prefixes to Document, DocumentIteraror, Decimal128, and DBPointer.
Also prefixed MinKey and MaxKey for consistency, though they are internal types.

[codecov-commenter]

Codecov Report

Merging #482 into master will increase coverage by 0.02%.
The diff coverage is 84.29%.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
+ Coverage   76.86%   76.89%   +0.02%     
==========================================
  Files         125      125              
  Lines       13000    13012      +12     
==========================================
+ Hits         9993    10005      +12     
  Misses       3007     3007              

Impacted Files	Coverage Δ
Sources/MongoSwift/BSON/CodingStrategies.swift	37.50% <ø> (ø)
Sources/MongoSwift/ChangeStream.swift	30.64% <0.00%> (ø)
Sources/MongoSwift/ChangeStreamOptions.swift	40.00% <ø> (ø)
Sources/MongoSwift/Hint.swift	53.84% <0.00%> (ø)
Sources/MongoSwift/MongoClient.swift	87.09% <0.00%> (ø)
...ces/MongoSwift/MongoCollection+ChangeStreams.swift	43.75% <ø> (ø)
...ces/MongoSwift/MongoCollection+FindAndModify.swift	98.61% <ø> (ø)
Sources/MongoSwift/MongoCredential.swift	100.00% <ø> (ø)
...oSwift/Operations/CommitTransactionOperation.swift	0.00% <0.00%> (ø)
...goSwift/Operations/CreateCollectionOperation.swift	100.00% <ø> (ø)
... and 85 more

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[patrickfreed]

As part of this, we'll need to update the various guides and examples in our documentation to refer to the new types. The web framework / mongodb docs examples may need to be updated as well.

[kmahar]

I'm making a pass through all of our guides right now and it's turning into a rather significant endeavor as a lot of stuff is outdated, so I'll handle that in a separate PR

[kmahar]

update: put up #486 containing guides and examples changes.