impl From/Into Lua for char

[tkr-sh]

FromLua:

• integer: check if u8, if yes convert to char else error
• string: only if one char the only char, else error
• else: error

IntoLua:
convert to a ::mlua::String of length 1

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Why ?
I think its important to have support for char since it's in std

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Note

char in rust is more a u32 than a u8 actually. So Im ok that this is modified from u8 to u32 if needed.

[khvzak]

Thanks