Context

Invasively use the flag for all error-returning functions called by merge_types
Consequently have faster fast errors (fixes a few cases where we still produced expensive errors where we shouldn't have)
GADTify the flag to have static guarantees that fast errors are always caught
reduce usage of catch-all for non-fast errors

Manually testing the MR

Document the interface of any function added or modified (see the coding guidelines)
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Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
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Edited Jan 03, 2022 by Mehdi Bouaziz