runtime: remove Default req on account scan interfaces

[t-nelson]

Problem

the accounts scanning interfaces have a seemingly unnecessary Default requirement, which restricts what types can be used to collect in to.

Summary of Changes

switch to FnMut closures and move the accumulator construction out to the call sites

[t-nelson]

first commit is the interesting one. second is straightforward from there

[Lichtso]

Might be more effort than it is worth, but can't scan_accounts() return an impl Iterator? Then we could simply collect into a container at the call site.

[jeffwashington]

lgtm

Pull request has been modified.

[t-nelson]

Might be more effort than it is worth, but can't scan_accounts() return an impl Iterator? Then we could simply collect into a container at the call site.

i'd considered that but a couple of its sibling methods have some extra logic and i opted for a consistent api across them instead