Expand on strings

[Varriount]

• Mention value semantics
• Go over slicing vs substr
• Explain using unicode module
• Mention internal representation (length prefixed with null terminator for literals)

[timotheecour]

http://nim-by-example.github.io/strings/ also has issues:
Proc Strings: raw strings, but the method name that prefixes the string is called [unfinished sentence]
echo r" "" " should output ' " ' but the output shown is ' "'
should show difference between:
echo re "\b[a-z]++\b"
echo re"\b[a-z]++\b"

[flaviut]

• Mention value semantics
• Go over slicing vs substr
• Explain using unicode module
• Mention internal representation (length prefixed with null terminator for literals)

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@timotheecour Done, thank you for pointing those errors out, they're a little embarrassing.