Add string builder helper (#2999)

This change adds a new string builder type, that can be used to construct
strings internally by appending various types of other strings.

JerryScript-DCO-1.0-Signed-off-by: Dániel Bátyai dbatyai@inf.u-szeged.hu

jerry-core/ecma/base/ecma-globals.h

@@ -1380,6 +1380,34 @@ typedef struct
   lit_utf8_size_t long_utf8_string_length; /**< length of this long utf-8 string in bytes */
 } ecma_long_string_t;
 
+/**
+ * String builder header
+ */
+typedef struct
+{
+  lit_utf8_size_t current_size; /**< size of the data in the buffer */
+} ecma_stringbuilder_header_t;
+
+/**
+ * Get pointer to the beginning of the stored string in the string builder
+ */
+#define ECMA_STRINGBUILDER_STRING_PTR(header_p) \
+  ((lit_utf8_byte_t *) (((ecma_string_t *) header_p) + 1))
+
+/**
+ * Get the size of the stored string in the string builder
+ */
+#define ECMA_STRINGBUILDER_STRING_SIZE(header_p) \
+  ((lit_utf8_size_t) (header_p->current_size - sizeof (ecma_string_t)))
+
+/**
+ * String builder handle
+ */
+typedef struct
+{
+  ecma_stringbuilder_header_t *header_p; /**< pointer to header */
+} ecma_stringbuilder_t;
+
 /**
  * Abort flag for error reference.
  */