builtin: Implement builtin_ascii

[corona10]

Implement builtin_ascii

[corona10]

@ncw
Please take a look!

[codecov-io]

Codecov Report

❌ Patch coverage is 96.15385% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.63%. Comparing base (c5b8c68) to head (a3dfc79).

Files with missing lines	Patch %	Lines
builtin/builtin.go	75.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## master #66 +/- ##
==========================================
+ Coverage 68.45% 68.63% +0.18% 
==========================================
Files 59 59 Lines 10495 10512 +17 ==========================================
+ Hits 7184 7215 +31 + Misses 2804 2789 -15 - Partials 507 508 +1 

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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!

funcstringEscape(a py.String) string {
s:=string(a)
varout bytes.Bufferfor_, c:=ranges {
switch {
casec<0x20:
switchc {
case'\t':
out.WriteString(`\t`)
case'\n':
out.WriteString(`\n`)
case'\r':
out.WriteString(`\r`)
default:
out.WriteRune(c)
}
casec<0x100:
out.WriteRune(c)
casec<0x10000:
fmt.Fprintf(&out, "\\u%04x", c)
default:
fmt.Fprintf(&out, "\\U%08x", c)
}
}
returnout.String()
}

[ncw]

LGTM - thank you - please merge :-)