Statically sized strings

[kryptan]

There is currently a gap in the language - there are no statically sized strings:

Vec<T>	String
[T; N]
&[T]	&str

#339 has added statically sized byte literals but not strings because there is no corresponding type. This RFC also contains motivation for statically sized strings.

[petrochenkov]

Related issue - #1179 (suggests a way to introduce fixed strings backward compatibly).

[Centril]

Closing in favor of #2460.