Add FxVector type to Typed Racket

The `for` forms for fxvectors are future work
Add in-flvector, in-fxvector to TR base environment
scribble/text: fix contract and docs on `with-writer`
scribble/html: add 'index flag in `url-roots`

This flag can be used to make "file://..."-rendered pages easier
to inspect.
Make flvectors and fxvectors sequences.
Simplify types for flvector and fxvector operations.
scribble/manual: delay expressions that require collection-based files

This changed make `(require scribble/manual)` work in an executable,
although actually rendering documents requires the "scribble"
collection.
Make flvectors and fxvectors subtypes of sequences.
Fix TR optimizer test.
Improvement and bug fixes to the new class/c; try again to re-enable it

Fixed:
- class-field-accessor and class-field mutator,
- the interaction between init args contracts and subclasses,
- object=? on wrapped objects (ie if 'this' flows out and
  is compared to the object thatn 'new' returns)
- contract-name for class/c

Also, the code now uses impersonator properties internally to track a
class that is wrapped (instead of a wrapper struct).  This simplifies
a bunch of places in the code and paves the way for classes that have
a class/c contract to be chapeone-of the original class (altho that
doesn't work yet)
remove dead code

and shrink dependencies correspondingly
fix class/c contract-name implementation

specifically, fix the case where a field doesn't have
a contract, but just has to exist
track test case names to help find infinite loops
more unexpected strange things broke, so disable new class/c yet again

No joy in mudville. See
http://drdr.racket-lang.org/28175/ for details

Also fix (just introduced) infinite loop in class/c contract-name
implementation and tweak naming stuff a little so tests pass
Add the random generation bechmark.

For now it's in the examples directory.
typos and notes

Thanks to Eli.
Allow prefix function arrow type

Also fix a regression that made arrow type
parsing more permissive than desired due to
missing colon^s
Add ->* type constructor for optional arguments
Abstract out function that checks ->* style types
Lift out `partition-kws` function into util module