Fix heap over-allocation in calculate_heap_usage

Function was using num_bytes in SMALL_BIG_EXT case when estimating heap
consumption: the correct input is roughly `num_bytes / sizeof(digit) +
align_padding`.

Also add `intn_required_digits_for_unsigned_integer` for this purpose to
intn.h.

This change required newly introduced `size_align_up_pow2` in utils.h.

Signed-off-by: Davide Bettio <davide@uninstall.it>

Author: bettio

src/libAtomVM/externalterm.c

@@ -1011,9 +1011,10 @@ static int calculate_heap_usage(const uint8_t *external_term_buf, size_t remaini
             }
 
             // num_bytes > 8 bytes || uint64_does_overflow_int64
+            size_t required_digits = intn_required_digits_for_unsigned_integer(num_bytes);
             size_t data_size;
             size_t unused_rounded_len;
-            term_intn_to_term_size(num_bytes, &data_size, &unused_rounded_len);
+            term_intn_to_term_size(required_digits, &data_size, &unused_rounded_len);
             return BOXED_INTN_SIZE(data_size);
         }
 

src/libAtomVM/intn.h

@@ -160,6 +160,11 @@ int intn_to_integer_bytes(const intn_digit_t in[], size_t in_len, intn_integer_s
 
 size_t intn_required_unsigned_integer_bytes(const intn_digit_t in[], size_t in_len);
 
+static inline size_t intn_required_digits_for_unsigned_integer(size_t size_in_bytes)
+{
+    return size_align_up_pow2(size_in_bytes, sizeof(intn_digit_t)) / sizeof(intn_digit_t);
+}
+
 static inline intn_integer_sign_t intn_negate_sign(intn_integer_sign_t sign)
 {
     return (sign == IntNPositiveInteger) ? IntNNegativeInteger : IntNPositiveInteger;