fix(beam): handle string binaries in Seq enumerator

[dbrattli]

Summary

• Fix badarg crash when applying Seq.X operations (e.g. Seq.mapi, Seq.map, Seq.toList) to a string on the BEAM target.
• F# strings are seq<char> by language definition, but Fable.Beam lowers them to Erlang binaries. fable_utils:get_enumerator/1 had no clause for binaries, so the fallback path (lists:flatten([Other])) wrapped the binary into a single-element list — the enumerator then handed the whole binary back as one "char", crashing later code paths (e.g. <<C/utf8>> from string c).
• Add a binary clause that converts to a Unicode codepoint list, matching the existing to_list/1 handling.

Minimal repro

"Hello" |> Seq.mapi (fun _ c -> string c) |> String.concat ""
// CLR/JS/Python: "Hello"
// BEAM (before): error:badarg in seq:enumerator_generate_while_some
// BEAM (after):  "Hello"

Stack trace before fix

seq:enumerator_generate_while_some/3-fun-2-     (seq.erl:372)
fable_utils:enumerate_to_list_loop/2            (fable_utils.erl:274)
fable_utils:enumerate_to_list/1                 (fable_utils.erl:268)
fable_string:join/2                             (fable_string.erl:171)

Test plan

• Quicktest reproduces the original crash before the fix; returns "Hello" after.
• Non-ASCII handled correctly: "über" enumerates as 4 codepoints (verified raw output <<"ü-b-e-r">>, 8 bytes), not 5 bytes.
• New regression tests in tests/Beam/StringTests.fs covering Seq.mapi, Seq.map, Seq.toList, Seq.length over strings.
• Full BEAM suite green: 2441 passed, 0 failed.

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