This is a string type for compactly storing short strings of statically-known size. Each character is stored in one byte, so currently only the Latin-1 subset of Unicode is supported.
To use, call FixedSizeString{n}(itr), where n is the length and itr is an iterable
of characters. Alternatively, other string types can be converted to FixedSizeString{n}.
FixedSizeStrings works well in the following cases:
- Very short strings, e.g. <= 8 characters
 - Storing many strings of the same length, when the number of unique strings is large
 
If you have a large array with a relatively small number of unique strings, it is
probably better to use PooledArrays with whatever string type is convenient.
TODO and open questions:
- Support more characters by adding a parameter for the representation (UInt16, UInt32)
 - Does it make sense to support UTF-8?
 - Possibly add 
MaxLengthString, which is the same except can be padded with 0 bytes to represent fewer than the maximum possible number of characters.