foldl(2)/foldr(2): support openArray

[foldl]

foldl*(sequence, operation, first) supports openArray, so, it would be nice and consistant that foldl(2) also supports it. This PR does this.

But, I am not sure if this is an issue of the compiler (Nim Compiler Version 2.2.0 [Windows: amd64]):

let s = sequence   # when sequence is an openArray, compiler complains 

If it's an issue of the compiler, than this PR is not needed.

So as foldr(2).

[ringabout]

2025-01-21T09:47:44.2138650Z �[1m�[36mTest "tests/stdlib/tsequtils.nim" in 
2025-01-21T09:47:44.2142510Z Output:
2025-01-21T09:47:44.2142770Z Traceback (most recent call last)
2025-01-21T09:47:44.2143060Z tsequtils.nim(478)       tsequtils
2025-01-21T09:47:44.2143340Z system.nim(936)          concat
2025-01-21T09:47:44.2143620Z arc.nim(230)             nimDecRefIsLast
2025-01-21T09:47:44.2143980Z SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Test failures seem to be related

[foldl]

I found that let s = sequence is OK on devel branch when sequence is an openArray.
So this PR is not needed.

And, test failures are bugs of the compiler.

[foldl]

Oh, not exactly. Sign.