break contract-test.rktl into 51 files

(plus one new file to act as a driver for
running the whole test suite)

Beyond the obviousness of this cleanup, it also helps me
test the contract system without having to load any code
that depends on it (making the test cases far more useful
when I'm breaking low-level parts of the contract system
(like ->))
add myself to the email notification
Switch two type errors to internal errors

One of these was broken and falling into that case would
raise a random runtime error anyway.
Document contract-continuation-mark-key.
Move typed-racket-more to typed-racket-pkgs.
remove dependency on racket/match
Improve some texts.
Allow multiple extra files.
Delay running git, to avoid a premature warning about $GIT_DIR.
Explain how to build the pages.
add `identifier-bindig-symbol'; fix free-id-table for renames

The implement of `free-id-table' uses `identifier-binding'
to decide on a symbolic name as a key for each identifier,
but `identifier-binding' doesn't provide enough information
for local and top-level bindings. The new `identifier-binding-symbol'
function provides that information.

Closes PR 13911
change `find-relative-directories' to normalize completed paths

Avoids problems with clients (like the Scribble part of `raco setup')
that compare paths.
games docs: fix an absolute-path problem
contract test suite post-breakup misc cleanups
Fixed a typo: part of define-generics's documentation was in the wrong scope.
Keyword arguments to define-generics can be in any order, before/after methods.
Added tests for varying the order of keyword options to define-generics.
Documented the new argument order accepted by define-generics.
Removed #:prop-defined-already? option from private define-generics macro.

In the few cases that used this option, the only definition needed from the
private define-generics macro was gen:<name>.  This is easy to define directly
without using the macro, so I have changed the code to do so and avoid redundant
definitions of methods.
Renamed private version of define-generics to define-primitive-generics.