Replace PB_MANY_FIELDS with PB_FIELD_16BIT and PB_FIELD_32BIT.

This allows more precise control over the memory use vs. field size.

pb.h

@@ -33,6 +33,11 @@
 #define PB_MAX_REQUIRED_FIELDS 64
 #endif
 
+#if PB_MAX_REQUIRED_FIELDS < 64
+#warning You should not lower PB_MAX_REQUIRED_FIELDS from the default value (64). \
+         The automatic checks against too low value will not be active.
+#endif
+
 /* List of possible field types. These are used in the autogenerated code.
  * Least-significant 4 bits tell the scalar type
  * Most-significant 4 bits specify repeated/required/packed etc.
@@ -99,21 +104,29 @@ typedef enum {
 
 /* This structure is used in auto-generated constants
  * to specify struct fields.
- * You can change field sizes here if you need structures
+ * You can change field sizes if you need structures
  * larger than 256 bytes or field tags larger than 256.
  * The compiler should complain if your .proto has such
- * structures ("initializer too large for type").
+ * structures. Fix that by defining PB_FIELD_16BIT or
+ * PB_FIELD_32BIT.
  */
 typedef struct _pb_field_t pb_field_t;
 struct _pb_field_t {
 
-#ifndef PB_MANY_FIELDS
+#if !defined(PB_FIELD_16BIT) && !defined(PB_FIELD_32BIT)
     uint8_t tag;
     pb_type_t type;
     uint8_t data_offset; /* Offset of field data, relative to previous field. */
     int8_t size_offset; /* Offset of array size or has-boolean, relative to data */
     uint8_t data_size; /* Data size in bytes for a single item */
     uint8_t array_size; /* Maximum number of entries in array */
+#elif defined(PB_FIELD_16BIT) && !defined(PB_FIELD_32BIT)
+    uint16_t tag;
+    pb_type_t type;
+    uint8_t data_offset;
+    int8_t size_offset;
+    uint16_t data_size;
+    uint16_t array_size;
 #else
     uint32_t tag;
     pb_type_t type;