Export MapIterator

Optional fields (attic-labs#3287)

This adds optional fields to structs.

New version: 7.4

To create a struct type with optional fields use types.MakeStructType2

There are some API changes in this commit and there will be some more in followup commits.

Fixes attic-labs#2327

Remove OID (attic-labs#3275)

Fixes attic-labs#3121

Last commit where the Noms JS SDK is present

Write sequence read-ahead cache to correct index (attic-labs#3025)