bug#56641: Deprecate `lsh`

[Mattias Engdegård]

The function `lsh` works almost like `ash` but is much slower, cannot be 
constant-folded by the compiler, and above all has confusing semantics for 
historical reasons. Using `lsh` instead of `ash` is always a mistake.

For these reasons we should deprecate `lsh`. It is not important to remove it 
as soon as possible, but to dissuade people from accidentally using it in their 
code.