Update libfmt to a tagged release with complex support

[n-west]

While adding fft operators to bland (to support #54) I need to add at least some comlpex number support. The first thing I added was a repr() operator to just print out values because there's some complication going to numpy...

I had to update libfmt to a recent commit because complex support was added in Mar 13 (fmtlib/fmt#1467, commit: fmtlib/fmt@73f2b34#diff-f82ce183442c6e77ea3fe297e25d5393cc13067caeb951a36923cbdba0c5a36f)

Now we're just pointing to some random commit since then. When they tag a release upgrade the content we fetch to the released version.

[n-west]

11.0 seems to be tagged with that commit. I tested 11.0.2 and it works with no issues, but produces a lot of compiler warnings about unreachable loops. That's not such a big deal, but going to leave this alone until I can fineness