Improved optimization of CONCATENATE

Calls to CONCATENATE with (some) constant arguments can be simplified by combining some of the constants together. For example, currently

(disassemble (lambda () (concatenate 'string "foo" "bar"))

=>

; disassembly for (LAMBDA ())
; Size: 39 bytes. Origin: #x536DB40C ; (LAMBDA ())
; 0C: 498B4510 MOV RAX, [R13+16] ; thread.binding-stack-pointer
; 10: 488945F8 MOV [RBP-8], RAX
; 14: 488B15BDFFFFFF MOV RDX, [RIP-67] ; "foo"
; 1B: 488B3DBEFFFFFF MOV RDI, [RIP-66] ; "bar"
; 22: B904000000 MOV ECX, 4
; 27: FF7508 PUSH QWORD PTR [RBP+8]
; 2A: B8A2C93550 MOV EAX, #x5035C9A2 ; #<FDEFN SB-KERNEL:%CONCATENATE-TO-STRING>
; 2F: FFE0 JMP RAX
; 31: CC10 INT3 16

but these two string constants could be combined (the code would still have to copy that combined string, though.)