wanted: smaller out-of-line structure-instance-allocation

Every structure constructor constain code like the following inline. This is OK for inlined constructors, but for out-of-line versions it would be better to call an out-of-line allocator instead, and do only initialization inline in the constructor.

; disassembly for (LAMBDA ())
; 002D89E4: 48892C2528081020 MOV [#x20100828], RBP ; no-arg-parsing entry point
; 9EC: 4C8B1C25802B1200 MOV R11, [#x122B80] ; boxed_region
; 9F4: 498D4B70 LEA RCX, [R11+112]
; 9F8: 48390C25882B1200 CMP [#x122B88], RCX ; boxed_region
; A00: 763B JBE L2
; A02: 48890C25802B1200 MOV [#x122B80], RCX ; boxed_region
; A0A: 498D4B01 LEA RCX, [R11+1]
; A0E: L0: 48C741FF5A0D0000 MOV QWORD PTR [RCX-1], 3418
; A16: 48312C2528081020 XOR [#x20100828], RBP
; A1E: 7403 JEQ L1
; A20: 0F0B09 BREAK 9 ; pending interrupt trap
; A23: L1: 488B1566FFFFFF MOV RDX, [RIP-154] ; #<SB-KERNEL:LAYOUT
                                                              ; for PACKAGE ..>
; A2A: 48895107 MOV [RCX+7], RDX
; A2E: 488BD1 MOV RDX, RCX
; A31: 488BE5 MOV RSP, RBP
; A34: F8 CLC
; A35: 5D POP RBP
; A36: C3 RET
; A37: 0F0B0A BREAK 10 ; error trap
; A3A: 02 BYTE #X02
; A3B: 18 BYTE #X18 ; INVALID-ARG-COUNT-ERROR
; A3C: 54 BYTE #X54 ; RCX
; A3D: L2: 6A70 PUSH 112
; A3F: 4C8D1C25904C1100 LEA R11, [#x114C90] ; alloc_tramp
; A47: 41FFD3 CALL R11
; A4A: 59 POP RCX
; A4B: 488D4901 LEA RCX, [RCX+1]
; A4F: EBBD JMP L0