stdlib organization & documentation improvements

[metagn]

asyncmacro is converted to an import/export in asyncdispatch instead of an include, since async imports asyncmacro as well. This seems to work without extra changes?

jsfetch now exports jsformdata and jsheaders to reduce burden on users.

The following modules are added to doc/lib.md (note that some of these have messages like "unstable API" or "experimental", but these messages have been there for years, and some modules already in doc/lib.md have them as well):

• compilesettings, effecttraits (core)
• formatfloat*, objectdollar*, widestrs* (strings)
• reservedmem, exitprocs*, syncio*, appdirs**, cmdline**, dirs**, envvars**, files**, oserrors**, paths**, symlinks** (generic OS services)
• async, asyncmacro, socketstreams (internet protocols)
• isolation, tasks, typedthreads* (threading)
• genasts (generators)
• decls, importutils, wrapnils, assertions* (miscellaneous)
• jsfetch, jsre (javascript)
• db_odbc (database)
  * part of nimPreviewSlimSystem
  ** part of os module splitup

The following modules have changed sections:

• lenientops (core -> math)
• ropes, strtabs (strings -> collections)
• parseutils, pegs (strings -> parsers)
• marshal (generic OS services -> serialization (new section))
• jsonutils (parsers -> serialization (new section))

The following deprecated modules have been removed from documentation and moved to lib/deprecated/pure:

• sums (moved to nimble package)
• mersenne (graveyard)
• oswalkdir

The following modules have not been included in doc/lib.md for various reasons:

• jsformdata, jsheaders (essentially submodules of jsfetch)
• pathnorm (seems dubious)
• chains (too small/seems internal)
• experimental/diff (seems internal)
• rtarrays (says "not ready for other people to use")
• outparams (tied to experimental feature and seems temporary)
• prelude (non-essential, tiny module that has to be included to use)
• ssl_certs/ssl_config (are these useful for other people?)
• streamwrapper (straight up don't understand this module, it's internally used by osproc)
• sysatomics (deprecated in place of std/atomics which is included)

If you have any objections, please say so.

[github-actions]

Thanks for your hard work on this PR!
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