Add user friendly file IO functions

[Nicolas Abril]

#573 adds the primitive file IO functions for Bend, building on the IO functions that HVM exposes.
However, for the common use case they are a bit hard to use.

So we'd like more usuer-friendly functions. I propose these, but we could have others:

• IO/Fs/read_file: (path: String) -> (List U24), reads a file in its entirety and returns a list of bytes.
• IO/Fs/read_until_end: (file: U24) -> (List U24), takes a file descriptor as returned by IO/Fs/open and reads it until EOF.
• IO/Fs/read_line: (file: U24) -> (List U24), takes a file descriptor and reads a line until \n is found.
• IO/Fs/write_file: (path: String) -> (data: List U24) -> *, replaces the entire content of a file with some contents (or writes a new file).

Since reads have a maximum length imposed by hvm, the read functions should read in chunks and then stitch them together.

read_line can read a large chunk, check for \n, if found split the string, and then move back the tail length using seek. I think this will be a bit faster than reading char by char.

I think these functions should operate with lists of bytes, so they'll work with both binary file and encoded text files.