More support for different string types

[ajusa]

1. Fixed length strings. In order to read a string that is exactly 200 bytes, you are forced to make a subparser and use @get. Ideally, we can define a new single letter type that stands for a fixed string length, and it is always 200 bytes long.
2. Support for other string encodings. Maybe have s16 mean utf-16? This also isn't too bad to implement using getters and setters, but for really common encodings it might be worth including them straight into binarylang itself.