RUST-314 Remove constants for subtypes and element types

[saghm]

No description provided.

[patrickfreed]

While looking at this, it got me thinking that we might want to revisit the names used here and in Bson.

A couple of things come to mind:

• FloatingPoint: when surveying other drivers, all but Ruby (which doesn't have a explicit type for Bson but instead extends existing Float) refer to this as "Double". I know there isn't a double type in Rust, but the same is true for Go, and they still refer to the BSON type as a "double".
• NullValue can probably be just Null like the Bson enum case
• Integer32Bit / Interger64Bit / Decimal128Bit can be Int32 / Int64 / Decimal128 respectively
  • Usually I'm for verbosity over terseness, but in this case the verbosity actually hurts discoverability imo
  • Given we're in Rust, maybe it would make sense to use I32 / I64 like the Bson enum case names
• TimeStamp I think is usually just Timestamp with no capital "S"

One other oddity is that for strings, the Bson enum case is String and the type is Utf8String, but for datetimes, the Bson case and type case are both UtcDatetime. The UTC requirement is expressed at the type level by defining the case's associated value as a DateTime<Utc>, so the "Utc" prefix is redundant and, imo, can be dropped.

All this feedback isn't exactly related to the ticket here, so I'm fine with not addressing it here. I just figured this was a decent spot to mention it.

[saghm]

Given that this ticket is removing them from the public API, I'm not overly concerned about the names of these constants as long as they're easy to understand. I'm not strictly opposed to renaming the Bson enum variants though; want to file a ticket for that?

[patrickfreed]

Yeah by "here" I actually meant the ElementType enum below. Filed RUST-345.

[saghm]

Ah, sorry for misunderstanding. Thanks for filing the ticket!