Choose/Oneof: shrink towards the first or not

[sir4ur0n]

Followup of this discussion

@c-cube asked if we really want to shrink towards the first element of a list of values/generators (choose and oneof families of functions).

IMHO it's best to shrink towards the first element of the list:

• consistency: no matter the starting point (root generated value), the "optimal" shrink will always be the same
• it doesn't hurt either, even if no branch is smaller than another, to always pick one

We can also provide an optional parameter to decide the strategy: first element, or none (only shrink on the selected one)

Discuss 😁

[c-cube]

I think it's fine to shrink towards the beginning of the list, if it's clearly documented :-). It's a good ability to have, I think I was just surprised by it initially.

[c-cube]

I think other discussions on #116 and previous have converged to a point where this is indeed the better behavior.