Wrap form inputs in a span for compatibility with browser extensions

[craiglyons]

Problem: The form example does not work with the 1Password browser extension in Chrome and Firefox. It may also be affected by other browsers / extensions.

Solution: Wrap inputs in a span element. This is applied to the function that generates the input elements, which gives the fix a very small footprint.

Related to: https://github.com/elm/virtual-dom/issues/147 https://github.com/evancz/guide.elm-lang.org/pull/185