vm: add OP_GETIDX0 for fast array[0] access

Fuses MOVE+LOADI_0+GETIDX pattern into single instruction.
Saves 4 bytes per arr[0] access (7 bytes -> 3 bytes).

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/ops.h

@@ -48,6 +48,7 @@ OPCODE(SETMCNST,   BB)       /* R[a+1]::Syms[b] = R[a] */
 OPCODE(GETUPVAR,   BBB)      /* R[a] = uvget(b,c) */
 OPCODE(SETUPVAR,   BBB)      /* uvset(b,c,R[a]) */
 OPCODE(GETIDX,     B)        /* R[a] = R[a][R[a+1]] */
+OPCODE(GETIDX0,    BB)       /* R[a] = R[b][0]; a+1 for method call */
 OPCODE(SETIDX,     B)        /* R[a][R[a+1]] = R[a+2] */
 OPCODE(JMP,        S)        /* pc+=a */
 OPCODE(JMPIF,      BS)       /* if R[a] pc+=b */