Use inline to vastly simplify the exposed API. This allows a
comptime-known endian parameter to be propogated, making extra functions
for a specific endianness completely unnecessary.
Most of this migration was performed automatically with `zig fmt`. There
were a few exceptions which I had to manually fix:
* `@alignCast` and `@addrSpaceCast` cannot be automatically rewritten
* `@truncate`'s fixup is incorrect for vectors
* Test cases are not formatted, and their error locations change
* 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
