The bson.NewEncoder and bson.NewDecoder functions (and New*ValueWriter/New*DocumentReader functions) currently return an error that only informs the user that they've passed in a nil ValueWriter or ValueReader, respectively. That error makes the API awkward to use and could be replaced by a panic because there's typically no way to recover from a missing ValueWriter or ValueReader (i.e. that's an implementation error, not a runtime error). Remove the returned errors to make the API nicer to use.

Definition of done:

• Remove the error returned from:
  • bson.NewEncoder
  • bson.NewDecoder
  • Decoder.Reset
  • Decoder.SetRegistry
  • Decoder.SetContext (Deprecated)
  • Encoder.Reset
  • Encoder.SetRegistry
  • Encoder.SetContext (Deprecated)
• (Optional) Explicitly panic if the ValueWriter or ValueReader argument is nil.
  Note that the alternative is to let subsequent calls to Encode/Decode panic with a "nil pointer dereference" (which is what the Go "encoding/json" library does). An explicit panic may be easier to understand, but the user will get a panic one way or another if they pass nil.
• Remove the error returned from bson.NewBSONValueWriter and bson.NewExtJSONValueWriter.
• Rename bson.NewBSONValueWriter to bson.NewValueWriter.
• Rename NewBSONDocumentReader to NewValueReader.
• Remove NewBSONValueReader; do not support creating single-value-reader decoders (that's what UnmarshalValue is for).
• (Optional) Panic if the io.Writer or io.Reader argument is nil.