Adds fixed-length unicode String encoding

[bogovicj]

As specified by Zarr 2

see saalfeldlab/n5-zarr#74

[bogovicj]

@bogovicj (note to self)
https://github.com/zarr-developers/zarr-extensions/blob/main/codecs/vlen-utf8/README.md#format-and-algorithm

In the encoded format, each chunk is prefixed with a 32-bit little-endian unsigned integer (u32le) that specifies the number of elements in the chunk. This prefix is followed by a sequence of encoded elements in lexicographical order. Each element in the sequence is encoded by a u32le representing the number of bytes followed by the bytes themselves. The bytes for each element are obtained by encoding the element as UTF8 bytes.