Add op code for 'return.

This is just short-hand for 'branch to end of function', but avoids
needing to patch the value after compiling the function.

Signed-off-by: Keith Packard <keithp@keithp.com>

snek-code.c

@@ -140,6 +140,8 @@ const char * const snek_op_names[] = {
 	[snek_op_range_start] = "range_start",
 	[snek_op_range_step] = "range_step",
 	[snek_op_in_step] = "in_step",
+	[snek_op_return] = "return",
+
 	[snek_op_null] = "null",
 	[snek_op_nop] = "nop",
 	[snek_op_line] = "line",
@@ -419,7 +421,6 @@ snek_code_finish(void)
 {
 	if (snek_compile_size == 0)
 		return NULL;
-	snek_code_patch_forward(0, snek_compile_size, snek_forward_return, snek_code_current());
 	snek_code_t *code = snek_alloc(sizeof (snek_code_t) + snek_compile_size);
 
 	if (code) {
@@ -1178,6 +1179,9 @@ snek_code_run(snek_code_t *code_in)
 					snek_id_del(id);
 				}
 				break;
+			case snek_op_return:
+				ip = snek_code->size;
+				break;
 			case snek_op_assert:
 				if (!snek_poly_true(snek_a)) {
 					snek_error_0("AssertionError");

snek-gram.ll

@@ -38,7 +38,7 @@ command		: @{ snek_print_val = snek_interactive; }@ stat
 	 		}@
 		  OP opt-formals CP COLON suite
 			@{
-				if (snek_compile[snek_compile_prev] == snek_op_forward)
+				if (snek_compile[snek_compile_prev] == snek_op_return)
 					snek_code_delete_prev();
 				else
 					snek_code_add_op(snek_op_null);
@@ -99,7 +99,7 @@ small-stats-p	: SEMI small-stat small-stats-p
 		;
 small-stat	: assign-expr
 		| RETURN ret-expr
-			@{ snek_code_add_forward(snek_forward_return); }@
+			@{ snek_code_add_op(snek_op_return); }@
 		| BREAK
 			@{ snek_code_add_forward(snek_forward_break); }@
 		| CONTINUE

snek.h

@@ -146,6 +146,7 @@ typedef enum {
 	snek_op_range_start,
 	snek_op_range_step,
 	snek_op_in_step,
+	snek_op_return,
 
 	snek_op_line,
 
@@ -157,7 +158,6 @@ typedef enum {
 } __attribute__((packed)) snek_op_t;
 
 typedef enum {
-	snek_forward_return,
 	snek_forward_break,
 	snek_forward_continue,
 	snek_forward_if,