* Extract Call ast node tag out of SuffixOp; parameters go in memory
   after Call.
 * Demote AsmInput and AsmOutput from AST nodes to structs inside the
   Asm node.
 * The following ast nodes get their sub-node lists directly following
   them in memory:
   - ErrorSetDecl
   - Switch
   - BuiltinCall
 * ast.Node.Asm gets slices for inputs, outputs, clobbers instead of
   singly linked lists

Performance changes:

throughput: 72.7 MiB/s => 74.0 MiB/s
maxrss: 72 KB => 69 KB (nice)