Build failure with nim 2.0.0
Hello, when I attempt to build the latest release, I get the following:
```
$ nim c -d:danger mosdepth.nim
Hint: used config file '/etc/nim/nim.cfg' [Conf]
Hint: used config file '/etc/nim/config.nims' [Conf]
Hint: used config file '/home/a/mosdepth/mosdepth-0.3.5/nim.cfg' [Conf]
......................................................................................................................................................................
/home/a/mosdepth/mosdepth-0.3.5/mosdepth.nim(261, 18) template/generic instantiation of `tag` from here
/home/a/.nimble/pkgs2/hts-0.3.25-c05c9adf6b2e10a7f07edfef29110ea14bb66611/hts/bam/auxtags.nim(63, 7) Warning: unreachable code after 'return' statement or '{.noReturn.}' proc [UnreachableCode]
/home/a/mosdepth/mosdepth-0.3.5/mosdepth.nim(261, 18) template/generic instantiation of `tag` from here
/home/a/.nimble/pkgs2/hts-0.3.25-c05c9adf6b2e10a7f07edfef29110ea14bb66611/hts/bam/auxtags.nim(66, 7) Warning: unreachable code after 'return' statement or '{.noReturn.}' proc [UnreachableCode]
/home/a/mosdepth/mosdepth-0.3.5/mosdepth.nim(261, 18) template/generic instantiation of `tag` from here
/home/a/.nimble/pkgs2/hts-0.3.25-c05c9adf6b2e10a7f07edfef29110ea14bb66611/hts/bam/auxtags.nim(74, 7) Warning: unreachable code after 'return' statement or '{.noReturn.}' proc [UnreachableCode]
/home/a/mosdepth/mosdepth-0.3.5/mosdepth.nim(504, 3) Error: undeclared identifier: 'shallow'
candidates (edit distance, scope distance); see '--spellSuggest': 
 (3, 4): 'alloc'
 (3, 4): 'dealloc'
 (3, 4): 'realloc'
```
This began once I upgraded to nim 2.0.0.