Nim: `encoding: ASCII` does not work on Windows
Cc @sealmove

On my machine, the error looks like this:

```
C:\temp\kaitai_struct\tests\spec\nim\texpr_str_encodings.nim(7) texpr_str_encodings
C:\temp\kaitai_struct\tests\compiled\nim\expr_str_encodings.nim(122) fromFile
C:\temp\kaitai_struct\tests\compiled\nim\expr_str_encodings.nim(50) read
C:\temp\kaitai_struct\runtime\nim\kaitai_struct_nim_runtime.nim(332) encode
C:\Users\pp\.choosenim\toolchains\nim-1.6.4\lib\pure\encodings.nim(505) convert
C:\Users\pp\.choosenim\toolchains\nim-1.6.4\lib\pure\encodings.nim(354) open
Error: unhandled exception: cannot find encoding ASCII [EncodingError]
```

As I see [in `encodings.nim:338-355`](https://github.com/nim-lang/Nim/blob/v1.6.4/lib/pure/encodings.nim#L338-L355), the encoding is resolved differently on Windows. On non-Windows systems, apparently `iconv` is used, but on Windows the code uses a [`winEncodings`](https://github.com/nim-lang/Nim/blob/v1.6.4/lib/pure/encodings.nim#L75) lookup table for name and the corresponding code page number, and there's no `ascii`.