change hashe--stringify to defsubst

This form just performs stringification so there is no sound reason to
make it a macro. Indeed as a macro it committed the sin of multiple
evaluation.

I must have made it a macro for performance reasons (premature or not),
which is why defsubst exists in elisp.
prevent multiple evaluation in hashe--setup
support no-error-p for hashe-get
intentation tweak
tweak args creation in hashe--test-fixture
docstring tweaks to please flycheck