Printer: less verbose variables printing

[AltGr]

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Pass along a bindlib context to allow the variable names to be altered only when disambiguation is needed. Partial fix to #240 (doesn't affect the backends, only the printer for the intermediate ASTs).

This also has the benefit of making the output of the tests much more stable.

[AltGr]

(also included a little bit of cleanup, like removing things like Format.fprintf fmt "%a")