Add buffered reads for disk_file-related file access

[tehKaiN]

Related to #242

Currently, using mini-std to read small parts of files is very slow since I/O operations on files aren't buffered.

AmigaOS has buffered file I/O functions, but they are only available since KS2.0, so that's not usable here. Bebbo's stdlib buffers file access on its own by doing larger read to dedicated cache buffer, and then doing subsequent small reads from that buffer. ACE should probably have similar feature, at least when Bartman's compiler is used (so a part of mini-std perhaps).

I'm not looking towards writing this kind of code, especially managing feof flag as well as skipping intermediate buffer when doing bigger reads than its size, so I wouldn't mind if someone else adds that.