Add blurb on fast mutation.

This function is helpful to copy a bson document but not all of the
elements. Equivalent to a removal of a key from a document (which we
do not currently allow in bson_t since it is append/overwrite only).

This function will copy the current field that iter is pointing at
to the next field in the bson_t. You can override the name if you
so choose.

These just happened to be missing.

pass bare variables to BSON_UINTxx_TO_BE macros as they may use its parameters several times

Adds assertion for 256 byte structure. Additional static assertions
ensure typed versions are <= 256.