formatting: misc fixes

[64]

This PR fixes various issues w.r.t formatting:

• Handles inf / nan in %f and %F
• Handles negatives in %f and %F
• Handles padding and justification in %f and %F
• Assumes precision is zero when . is present but no number follows (fixes managarm/mlibc#209)
• Assumes precision to be 6 when no . is present in %f and %F
• Allows precision when working with unsigned integers (fixes managarm/mlibc#182)

There is still a lot more work to be done, especially around printing floating point numbers.

The relevant tests are added in managarm/mlibc#254.

[Geertiebear]

LGTM, although in general we prefer braceless one-line if statements.