EJSON: Add tests for custom types
EJSON: Fix asymmetry in equals
EJSON: Provide default implementation of clone for custom types
EJSON: Provide default implementation of equals for custom types
Docs improvements for #1745

Text before an api_box does not look good as an introduction to that
api_box due to spacing.
Reorder table of contents
history update for #1745
Merge branch 'pr/1745' into devel