util: add list_map_opt to lift list_map over options

This is particularly useful to deal with TFun's second parameter
(optional list of arguments).
Represent functions without arguments in typsig

This is an incompatible API change.
It allows to distinguish the types of f() and
f(void), which are treated differently in several
places of the C standard.

Closes #8, thanks to Stephen Kell.
Fix over-quoting of compiler argument strings

Closes #7. Thanks to Stephen Kell.
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