introduce align keyword

* remove `@setGlobalAlign`
 * add align keyword for setting alignment on functions and
   variables.
 * loads and stores use alignment from pointer
 * memcpy, memset use alignment from pointer
 * add syntax for pointer alignment
 * slices can have volatile
 * add u2, i2 primitives
 * ignore preferred align and use abi align everywhere
 * back to only having alignOf builtin.
   preferredAlignOf is too tricky to be useful.
   See #432. Partial revert of
   e726925e80.

See #37

src/zig_llvm.cpp

@@ -713,11 +713,6 @@ void ZigLLVMAddModuleDebugInfoFlag(LLVMModuleRef module) {
     unwrap(module)->addModuleFlag(Module::Warning, "Debug Info Version", DEBUG_METADATA_VERSION);
 }
 
-unsigned ZigLLVMGetPrefTypeAlignment(LLVMTargetDataRef TD, LLVMTypeRef Ty) {
-    return unwrap(TD)->getPrefTypeAlignment(unwrap(Ty));
-}
-
-
 static AtomicOrdering mapFromLLVMOrdering(LLVMAtomicOrdering Ordering) {
     switch (Ordering) {
         case LLVMAtomicOrderingNotAtomic: return AtomicOrdering::NotAtomic;