Changes

matching: argo => arg_id
 (commit: 2f57af2)

matching: add a test for do_for_multiple_match flattening failure

Note: this is due to mk_alpha_env raising Cannot_flatten during
splitting/precompilation.
 (commit: 30bb39d)

matching: push simple types to flatten_pattern

Unfortunately since the function is exposed and used in translcore we
need to keep the generic one, and introduce a flatten_simple_pattern.
 (commit: f491929)

matching: flatten_simple_pattern does not raise
 (commit: e17f815)

matching: try => match with exception

This makes it clearer where the exception comes from.
 (commit: ddf93aa)

Changes
 (commit: f8bf8c3)