add ignorePromise to safely ignore promise

[jfrolich]

This ignore will be more strict than the standard ignore function because it will only match Promise.t<'a>

This helps to find potential bugs:

• Where there is no promise returned (found 2 cases in the tests that didn't need ignore)
• Where there is some partly applied function (ignore will happily accept, and nothing will happen)

Especially the second case has caused bugs for us so we eliminated the general ignore from our codebase.

I called the function ignorePromise so it doesn't shadow when opened. It might be ok to shadow, and then we have the slightly nicer Promise.ignore.

[ryyppy]

sounds like a legit usecase, just wondering if ignorePromise is a good name... what about done()?

[jfrolich]

sounds like a legit usecase, just wondering if ignorePromise is a good name... what about done()?

Done sounds great!