strings: Add apfl_string_blank() and doucment apfl_string_copy()

It's really easy to accidentally pass an uninitialized string as dst into
the copy function, which will result in an free() call to an arbitrary
pointer. Maybe it's a better idea to not deinit the dst string before
copying? The documentation at least makes it more clear and the new
apfl_string_blank() function makes it easy to create an empty string.

src/apfl.h

@@ -50,10 +50,17 @@ int apfl_string_view_cmp(struct apfl_string_view, struct apfl_string_view);
 #define apfl_string_cmp(a, b) apfl_string_view_cmp(apfl_string_view_from(a), apfl_string_view_from(b))
 #define apfl_string_eq(a, b) (apfl_string_cmp((a), (b)) == 0)
 
+struct apfl_string apfl_string_blank(void);
 void apfl_string_deinit(struct apfl_string *);
-bool apfl_string_copy(struct apfl_string *dst, struct apfl_string_view src);
 struct apfl_string apfl_string_move(struct apfl_string *src);
 
+/**
+ * Copies a string from src to dst. dst must point to a blank string.
+ * Returns true on success, false otherwise (if the necessary memory could not
+ * be allocated).
+ */
+bool apfl_string_copy(struct apfl_string *dst, struct apfl_string_view src);
+
 struct apfl_string_builder {
     char *bytes;
     size_t len;

src/parser_test.c

@@ -111,7 +111,7 @@ expect_error_of_type(struct parser_test *pt, enum apfl_error_type want)
 static struct apfl_string
 new_string(struct parser_test *pt, const char *in)
 {
-    struct apfl_string out = { .bytes = NULL, .len = 0 };
+    struct apfl_string out = apfl_string_blank();
     if (!apfl_string_copy(&out, apfl_string_view_from(in))) {
         test_fatalf(pt->t, "Failed copying string in new_string");
     }

src/strings.c

@@ -54,12 +54,20 @@ apfl_string_view_cmp(struct apfl_string_view a, struct apfl_string_view b)
     return a.len > b.len ? 1 : -1;
 }
 
+struct apfl_string
+apfl_string_blank(void)
+{
+    return (struct apfl_string) {
+        .bytes = NULL,
+        .len = 0,
+    };
+}
+
 void
 apfl_string_deinit(struct apfl_string *string)
 {
     free(string->bytes);
-    string->len = 0;
-    string->bytes = NULL;
+    *string = apfl_string_blank();
 }
 
 bool
@@ -80,8 +88,7 @@ struct apfl_string
 apfl_string_move(struct apfl_string *src)
 {
     struct apfl_string out = *src;
-    src->bytes = NULL;
-    src->len = 0;
+    *src = apfl_string_blank();
     return out;
 }