memset and memcpy builtins should have alignment parameter

[andrewrk]

Alignment must be known at compile time. If the source and destination pointers are known to be aligned to a value that is greater than 1, this can allow more optimal code generation. Otherwise, an alignment of 1 will still work fine. Alignment must be a power of 2.

Additionally, the existence of this parameter will encourage users to use @import("std").mem.copy and @import("std").mem.set rather than unsafe low level intrinsics, and those who use these builtins will think more carefully when they decide to use it.

http://llvm.org/docs/LangRef.html#llvm-memcpy-intrinsic

[andrewrk]

Solved by #37