builtin: Implement builtin_ascii

[corona10]

Implement builtin_ascii

[corona10]

@ncw
Please take a look!

[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 0.01%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   68.61%   68.63%   +0.01%     
==========================================
  Files          59       59              
  Lines       10499    10512      +13     
==========================================
+ Hits         7204     7215      +11     
- Misses       2788     2789       +1     
- Partials      507      508       +1

Impacted Files	Coverage Δ
py/string.go	87.17% <100%> (+0.23%)	⬆️
builtin/builtin.go	79.55% <75%> (-0.07%)	⬇️

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[ncw]

See inline for adapting an existing function to do this

[corona10]

@ncw
Thanks for the kind review.
I 've added additional tests.
But I didn't reflect some of your reviews that I 've not understood yet.
I left comments on that!
Please take a look.

[corona10]

I've found this code does not work well.

>>> ascii('\U+10001')
"'\\U00010001'"

[corona10]

@ncw
I've added StringEscape which can be used for repr or ascii.
Note that StringEscape which need to ascii is below code.

If there is something I missed. Please let me know!

func stringEscape(a py.String) string {
    s := string(a)
    var out bytes.Buffer
    for _, c := range s {
        switch {
        case c < 0x20:
            switch c {
            case '\t':
                out.WriteString(`\t`)
            case '\n':
                out.WriteString(`\n`)
            case '\r':
                out.WriteString(`\r`)
            default:
                out.WriteRune(c)
            }
        case c < 0x100:
            out.WriteRune(c)
        case c < 0x10000:
            fmt.Fprintf(&out, "\\u%04x", c)
        default:
            fmt.Fprintf(&out, "\\U%08x", c)
        }
    }
    return out.String()
}

[ncw]

LGTM - thank you - please merge :-)