vm: add OP_RETTRUE and OP_RETFALSE for returning boolean literals

Add single-byte opcodes for returning true/false directly, completing
the set of literal return opcodes (RETSELF, RETNIL, RETTRUE, RETFALSE).

Codegen applies peephole optimization to fuse LOADTRUE/LOADFALSE + RETURN.

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

include/mruby/ops.h

@@ -75,6 +75,8 @@ OPCODE(RETURN,     B)        /* return R[a] (normal) */
 OPCODE(RETURN_BLK, B)        /* return R[a] (in-block return) */
 OPCODE(RETSELF,    Z)        /* return self */
 OPCODE(RETNIL,     Z)        /* return nil */
+OPCODE(RETTRUE,    Z)        /* return true */
+OPCODE(RETFALSE,   Z)        /* return false */
 OPCODE(BREAK,      B)        /* break R[a] */
 OPCODE(BLKPUSH,    BS)       /* R[a] = block (16=m5:r1:m5:d1:lv4) */
 OPCODE(ADD,        B)        /* R[a] = R[a]+R[a+1] */