Consider using __asan_default_options instead of ASAN_OPTIONS

[mschwager]

ASan flags can be specified in a number of ways: https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags.

Currently, we use the ASAN_OPTIONS ENV variable in a number of places:

ruzzy/README.md

Lines 57 to 70 in be40045

	```bash
	export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"
	```
	<details>
	<summary>Understanding these options isn't necessary, but if you're curious click here.</summary>
	### `ASAN_OPTIONS`
	1. Memory allocation failures are common and low impact (DoS), so skip them for now.
	1. Like Python, the Ruby interpreter [leaks data](https://github.com/google/atheris/blob/master/native_extension_fuzzing.md#leak-detection), so ignore these for now.
	1. Ruby recommends [disabling sigaltstack](https://github.com/ruby/ruby/blob/master/doc/contributing/building_ruby.md#building-with-address-sanitizer).
	</details>

ruzzy/Dockerfile

Lines 23 to 26 in be40045

	# 1. Skip memory allocation failures for now, they are common, and low impact (DoS)
	# 2. The Ruby interpreter leaks data, so ignore these for now
	# 3. Ruby recommends disabling sigaltstack: https://github.com/ruby/ruby/blob/master/doc/contributing/building_ruby.md#building-with-address-sanitizer
	ENV ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"

The downside of this approach is that we require an additional step from users to export this ENV variable. If we use the __asan_default_options function in our source code, then that's one less step. We still need to confirm that ASAN_OPTIONS can overwrite __asan_default_options at runtime in case users need different options.

[mschwager]

This is a bit more challenging than I initially anticipated due to the way Ruzzy is structured. When fuzzing C extensions, there ends up being two C extensions: the Ruzzy C extension and the target C extension. The Ruzzy C extension is the one we have the most control over, but I believe __asan_default_options would have to be defined in the target C extension. This certainly isn't impossible, but it's more work than adding __asan_default_options to cruzzy.c.