SWIFT-835 Standardize on conversion method names

[kmahar]

Renames all public and internal methods of the form asX to toX.

Relatedly I remembered while doing this that we had decided in the BSON design doc we would switch to using toX methods for BSON interop with other types, rather than adding initializers to types we don't own. But I forgot to add that to the list Neal did in #477. I went ahead and moved the BSONBinary -> UUID logic into BSONBinary.toUUID, and the BSONRegularExpression -> NSRegularExpression logic into BSONRegularExpression.toNSRegularExpression.

[codecov-commenter]

Codecov Report

Merging #484 into master will decrease coverage by 0.00%.
The diff coverage is 94.17%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
- Coverage   76.87%   76.86%   -0.01%     
==========================================
  Files         125      125              
  Lines       13003    12999       -4     
==========================================
- Hits         9996     9992       -4     
  Misses       3007     3007              

Impacted Files	Coverage Δ
...MongoSwiftSyncTests/SpecTestRunner/FailPoint.swift	7.35% <0.00%> (ø)
...sts/MongoSwiftSyncTests/SpecTestRunner/Match.swift	37.25% <0.00%> (ø)
...ts/MongoSwiftSyncTests/SyncChangeStreamTests.swift	26.82% <0.00%> (ø)
Tests/MongoSwiftSyncTests/SyncTestUtils.swift	60.20% <0.00%> (ø)
Sources/MongoSwift/MongoError.swift	60.69% <66.66%> (ø)
Sources/MongoSwift/BSON/BSON.swift	80.59% <100.00%> (ø)
Sources/MongoSwift/BSON/BSONDecoder.swift	66.18% <100.00%> (ø)
Sources/MongoSwift/BSON/BSONEncoder.swift	64.54% <100.00%> (ø)
Sources/MongoSwift/BSON/BSONValue.swift	76.00% <100.00%> (-0.17%)	⬇️
Sources/MongoSwift/MongoCollection+BulkWrite.swift	85.62% <100.00%> (ø)
... and 16 more

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