When a tag is serializable but not `write'--`read' invariant,
then it needs to be serialized and deserialized.
Also, clarify and check in `tag?' that a tag should be
serializable.
The `xref' produced by `setup/xref' uses the database to delay
loading "out.sxref"s, which cuts 64-bit DrRacket's initial
footprint by around 50MB (i.e., about 20%).
Also, add 'lsquo as allowed content.
Omitting the ` conversion in the first place was over-conservative.
There's a backward-compatibility issue with this addition (i.e., a
document might contain a backquote in a decoded context that is
meant to be rendered as a backquote), but the potential problems
seem minor.
For example, if you make a multi-column table with a
`racketblock' in each column, then the columns size
to fit the code --- instead of forcing the table width
to match the page width and forcing each column to take
an equal share width.
