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]

• [x] Mention value semantics
• [ ] Go over slicing vs substr
• [ ] Explain using unicode module
• [x] 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.