rework fuzzing API

The previous API used `std.testing.fuzzInput(.{})` however that has the
problem that users call it multiple times incorrectly, and there might
be work happening to obtain the corpus which should not be included in
coverage analysis, and which must not slow down iteration speed.

This commit restructures it so that the main loop lives in libfuzzer and
directly calls the "test one" function.

In this commit I was a little too aggressive because I made the test
runner export `fuzzer_one` for this purpose. This was motivated by
performance, but it causes "exported symbol collision: fuzzer_one" to
occur when more than one fuzz test is provided.

There are three ways to solve this:

1. libfuzzer needs to be passed a function pointer instead. Possible
   performance downside.

2. build runner needs to build a different process per fuzz test.
   Potentially wasteful and unclear how to isolate them.

3. test runner needs to perform a relocation at runtime to point the
   function call to the relevant unit test. Portability issues and
   dubious performance gains.

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