Patch 0001 fixes SRFI-69 bugs as specified. Patches 0002 to 0005 implement
the module (ice-9 generic-hash-tables) and then reimplement SRFI-69 and
(rnrs hashtables) and add an implementation of SRFI 126, all of them using
generic-hash-tables (and therefore compatible to each other).

I've called `make check' after each commit and all tests PASS: or XFAIL:,
except the four tests in linker.test, as reported in Bug 33991[1].

[1] https://debbugs.gnu.org/cgi/bugreport.cgi?bug=33991