`field-space=tight-decl` should apply to signature items as well

[nojb]

The option field-space=tight-decl should also apply to signature items, which are very similar to record fields, eg:

val foo: ty

instead of

val foo : ty

(by the way this is a common style, found eg in the OCaml compiler)

[nojb]

(I can look into implementing this if there is a consensus.)