sequtils: fix errors from `strictFuncs` use (#18998)

Nim 1.4.x compiled the below code without error when using
`--experimental:strictFuncs`

    import std/sequtils

    type Foo = ref object

    let foo1 = Foo()
    let foo2 = Foo()
    let foos = @[foo1, foo2]
    let fooTuples = @[(foo1, 1), (foo2, 2)]

    discard repeat(foo1, 3)
    discard zip(foos, foos)
    discard unzip(fooTuples)

However, since 2020-12-09, devel Nim produced errors like

    /tmp/bar.nim(11, 15) template/generic instantiation of `repeat` from here
    /foo/nim/pure/collections/sequtils.nim(172, 6) Error: 'repeat' can have side effects
    an object reachable from 'x' is potentially mutated
    /foo/nim/pure/collections/sequtils.nim(183, 15) the mutation is here
    /foo/nim/pure/collections/sequtils.nim(183, 15) is the statement that connected the mutation to the parameter

This commit reverts some `proc` to `func` changes so that code that:

- calls `repeat`, `zip`, or `unzip`
- and instantiates them with types containing `ref`

can once again be compiled with `strictFuncs`. Otherwise, a user might
be forced to drop or alter their `strictFuncs` use when upgrading from
Nim 1.4.x, or when writing new code that uses these procedures (at least
for now, with the current `strictFuncs` implementation).

This commit also adds tests to assert that the remaining funcs in this
module can be compiled with `strictFuncs` when used with types
containing `ref`.

The original batch of `proc` to `func` changes in `sequtils.nim` was in
commit 6f57ebae349f, which was partially reverted in 38eb021f8158.

See also: https://github.com/nim-lang/Nim/issues/16305

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