Support arguments to native closures
Minor refactoring of Obj-C indexer and tests

What was called 'headers' is actually a list of any arguments to libclang. It's
now splitted by space in the Indexer to allow multiple arguments (nevertheless,
libclang doesn't support indexing more than one file).

Also test method extracted to allow easier testing outside of the usual
".h+.m+.kt" framework
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