add encoding.iconv

[kbkpbot]

Module iconv provides functions convert between vstring(UTF8) to/from different encodings.

assert iconv.vstring_to_encoding('V大法好', 'GB18030' ) == [u8(86), 180, 243, 183, 168, 186, 195]
assert iconv.encoding_to_vstring([u8(86), 180, 243, 183, 168, 186, 195], 'GB18030') == 'V大法好'

[kbkpbot]

CI macOS show error

OK    [ 150/2006] C:   396.9 ms, R:     4.383 ms vlib/encoding/csv/writer_test.v
OK    [ 151/2006] C:   385.4 ms, R:     4.364 ms vlib/encoding/hex/hex_test.v
OK    [ 152/2006] C:   426.6 ms, R:     4.921 ms vlib/encoding/html/escape_test.v
 FAIL  [ 153/2006] C:  1644.2 ms, R:     0.000 ms vlib/encoding/iconv/iconv_test.v
>> compilation failed:
================== C compilation error (from clang): ==============
cc: ld: symbol(s) not found for architecture arm64
cc: clang: error: linker command failed with exit code 1 (use -v to see invocation)
... (the original output was 47 lines long, and was truncated to 2 lines)
===================================================================
(You can pass `-cg`, or `-show-c-output` as well, to print all the C error messages).
builder error: 
==================
C error found. It should never happen, when compiling pure V code.
This is a V compiler bug, please report it using `v bug file.v`,
or goto https://github.com/vlang/v/issues/new/choose .
You can also use #help on Discord: https://discord.gg/vlang .

OK    [ 154/2006] C:   461.2 ms, R:     7.025 ms vlib/encoding/leb128/leb128_test.v

It seems missing libiconv on macOS?

[spytheman]

It seems missing libiconv on macOS?

Yes, I will install it through brew on the CI.

[spytheman]

The windows gcc CI failure is not related to the PR.

[kbkpbot]

// write_file_encoding write_file convert `text` into `encoding` and writes to a file with the given `path`. If `path` already exists, it will be overwritten.
// For `encoding` in UTF8/UTF16/UTF32, if `bom` is true, then a BOM header will write to the file.
pub fn write_file_encoding(path string, text string, encoding string, bom bool) ! 

// read_file_encoding reads the file in `path` with `encoding` and returns the contents
pub fn read_file_encoding(path string, encoding string) !string 

[JalonSolov]

Making the BOM an option is probably the best way to go... when saving the data to a file. If you're writing a file specifically for use by something that doesn't need/want a BOM, then it makes sense to leave it out. Very handy to have, though if the file might be read by things that do know how to handle a BOM properly.

While a string is in in memory, it really doesn't do anything other than take up extra space. Your program should know what is in the strings it creates, so it doesn't need a BOM to tell it what format they are.

[spytheman]

I could not find anything more substantial to nitpick anymore.

Good work.