Minor `math/statistics' doc fixups
ffi/unsafe/custodian: fix weak-reference problem

Also fix a related problem in the `math' MPFR binding (which was
masked by the `ffi/unsafe/custodian' bug).
Fixing regression re path restriction patch
Improved performance of prime? for small numbers
Render `@hyperlink` in Markdown.

Render Scribble like

    @hyperlink["url" "content"]

as Markdown like

    [content](url)

Note that this only works for `@hyperlink`. The motivation is to
preserve content the author has explicitly written. (Previously,
`markdown-render.rkt` was discarding this; `text-render.rkt` still
does so.)

This does _not_ attempt to handle everything that `html-render.rkt`
would automatically generate and render as `<a>`. It simply can't --
things like hotlinked Racket keywords in code blocks simply won't work
in Markdown.
fix `quasisyntax' to better preserve syntax properties

Closes PR 13357
Render italic and bold in Markdown.

     Scribble => Markdown
  @bold{text}    **text**
@italic{text}     _text_
Improve ftp client. Add upload, progress monitor and something else.
net/ftp: make progress proc keyword, change progress protocol

The revised protocol for a progress procedure doesn't create
the thread automatically, and it provides an event to indicate
when the progress count changes.
Modernize cfg-parser from mzscheme to Racket libraries.
Add scribble/doclang2 with keyword support for customization, and documentation.
Add separate section to avoid conflict at scribble time.
raco setup: fix problem with doc index database

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.
Minor doc fix
Refactor to use syntax/id-table
Fix tests where type variable scoping is wrong
Simplify expansion of `pdefine:` and `define:`
Fix unsound type variable scoping

Closes PR 13123
Rackety
Explain scope of type variables in docs