check errors earlier in judgment-form

closes PR 13609
closes PR 13610
adjust docs for check-metafunction to make the example
use more representative

closes PR 13614
document define-metafunction's contract patterns properly

closes PR 13613
adjust commit 99ff0adbfda87791a99914b69caa0decef79cd04 to
be more friendly to the test suites
make check-metafunction and check-reduction-relation
pay attention to the contract/#:domain spec

closes PR 13616
Fix parsing of dotted formals.

Closes PR 13576.
windows: extflonum support

Enable extflonums in a MSVC build by relying on a MinGW-built DLL,
"longdouble.dll". The DLL is loaded on startup. If the DLL isn't
available, then `extflonum-available?' reports #f.

Instead of setting the floating-point mode globally to extended
precision, the mode is changed only just before (and restored right
after) extflonum arithmetic operations.
add a #:pre keyword to define-metafunction

The keyword allows the specification of contracts
that relate different arguments

closes PR 13617
fix header-file issues
avoid compiler warnings
fix non-extflonum build
pretty-format returns a String, not Void
Avoid stripping location information from syntax object in warning message.