Why use --gc:markAndSweep as default?
Is there a reason to use this over `refc` for Nimporter? Nimpy suggested compiler invocation makes no mention of it:

```
# Compile on Windows:
nim c --threads:on --app:lib --out:mymodule.pyd mymodule
# Compile on everything else:
nim c --threads:on --app:lib --out:mymodule.so mymodule
```

Additionally, microbenchmarking suggests is it is slower than `refc` (`nimr` is just an alias for `nim c -r --hints:off --verbosity:0`):

```
~/D/P/R/l/librtd-py ❯❯❯ nimr -d:danger --opt:speed --gc:markAndSweep --threads:on call_cli.nim; rm test.jsonl
Success: Analyzed RTD for 50 sequences totaling 1492590 bp in 19.9 seconds.
~/D/P/R/l/librtd-py ❯❯❯ nimr -d:danger --opt:speed --gc:refc --threads:on call_cli.nim; rm test.jsonl
Success: Analyzed RTD for 50 sequences totaling 1492590 bp in 15.1 seconds.
~/D/P/R/l/librtd-py ❯❯❯ nimr -d:danger --opt:speed --gc:arc --threads:on call_cli.nim; rm test.jsonl
Success: Analyzed RTD for 50 sequences totaling 1492590 bp in 12.5 seconds.
```