improve bit shift API in std.math
improve bit shift API in std.math

 * `shl` moved to `shlExact`
 * added `shl` and `shr` which are truncating like `<<` and `>>`.

closes #403

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