* add xz to std.compress
* prefer importing std.zig by file name, to reduce reliance on the
standard library being a special case.
* extract some types from inside generic functions. These types are the
same regardless of the generic parameters.
* expose some more types in the std.compress.xz namespace.
* rename xz.stream to xz.decompress
* rename check.Kind to Check
* use std.leb for LEB instead of a redundant implementation
