CircularIndex: fight against type promotion in size checks

To generate optimal code, size itself needs to be of the same type as
the index to avoid promotion to the largest type.

In full(), wrap the subtraction explicity in another type cast to avoid
another automatic type promotion.

src/hal/circular_buffer.h

@@ -12,11 +12,11 @@
 ///   (recommended to keep uint8_fast8_t as single byte operations are atomical on the AVR)
 /// @param size number of index positions.
 ///   It is recommended to keep a power of 2 to allow for optimal code generation on the AVR (there is no HW modulo instruction)
-template <typename index_t = uint_fast8_t, size_t size = 16>
+template <typename index_t = uint_fast8_t, index_t size = 16>
 class CircularIndex {
 public:
 #ifndef __AVR__
-    static_assert(size <= std::numeric_limits<index_t>::max(),
+    static_assert(size <= std::numeric_limits<index_t>::max() / 2,
         "index_t is too small for the requested size");
 #endif
 
@@ -33,7 +33,7 @@ public:
     inline bool full() const {
         // alternative without wrap-around logic:
         //   return tail != head && mask(tail) == mask(head);
-        return (head - tail) % (size * 2) == size;
+        return ((index_t)(head - tail) % (size * 2)) == size;
     }
 
     /// Advance the head index of the buffer.