Register hy.models.FString with hy-repr + tests.

[peaceamongworlds]

Currently hy-repr treats f-strings as a sequence, which produces unexpected results:

=> (hy-repr 'f"a{:a}")
'\'(, "a" (, :a))'

This makes it so that

=> (hy-repr 'f"a{:a}")
'\'f"a{:a}"'

[Kodiologist]

Note the typo hy-rep in NEWS.

[peaceamongworlds]

Note the typo hy-rep in NEWS.

Fixed.

[scauligi]

Make sure it handles conversions and other parameters correctly (and add corresponding tests):

=> (print (hy-repr 'f"a{:a =}"))
'f"a:a ={:a}"

[Kodiologist]

@peaceamongworlds Are you thinking of picking this back up soon? I'd like to make some long overdue changes to hy-repr, but I'd like to have these changes in first.

Superseded.