Accept Plain functions in every combinator

[gustavoguichard]

This PR introducing the tagging of composables at both type and runtime levels: fn.kind = 'composable'

This allows us to achieve: Composable<Composable<() => void>> === Composable<() => void>.

And by doing so we are now able to accept plain functions in any combinator, like so:

const add = (a: number, b: number) => a + b

// Before this PR:
const fn = collect({ add: composable(add), toString: map(composable(add), String) })

// With this PR:
const fn = collect({ add, toString: map(add, String) })

There are still 2 todos to my goal:

• [x] Make the branch resolver function be able to return plain functions instead of only composables or null
• [x] Make the applySchema work with plain functions and then we can deprecate withSchema because they will be virtually the same

I think this PR as is can be merged and the remaining tasks can come later as new features.