Review comments from pbkdf2.zig
Review comments from pbkdf2.zig

Move block definitions inside while loop.
Use usize for offset. (This still crashes on overflow)
Remove unneeded slice syntax.
Add slow test for Very large dkLen

raulgrell/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig