Add support for Guile 2.0.9.
* gash/compat.scm: New file.
* gash/compat/hash-table.scm: New file.
* Makefile.am: Add them.
* .dir-locals.scm: Add indentation for 'if-guile-version-below'.
* guix.scm (guile-2.0.9): New variable.
* gash/built-ins/echo.scm (escape->control): Use '#\esc'.
* gash/environment.scm: Use '(gash compat hash-table)'.
* gash/built-ins/break.scm,
gash/built-ins/cd.scm,
gash/built-ins/command.scm,
gash/built-ins/continue.scm,
gash/built-ins/dot.scm,
gash/built-ins/exec.scm,
gash/built-ins/exit.scm,
gash/built-ins/pwd.scm,
gash/built-ins/return.scm,
gash/built-ins/set.scm,
gash/built-ins/shift.scm,
gash/built-ins/trap.scm,
gash/built-ins/type.scm,
gash/built-ins/umask.scm,
gash/eval.scm,
tests/unit/automake.scm: Use '(gash compat)'.
squash! Add support for Guile 2.0.9.
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