Merge bitcoin/bitcoin#27929: Added static_assert to check that base_blob is using whole bytes.

5fc4939 Added static_assert to check that base_blob is using whole bytes. (Brotcrunsher)

Pull request description:

  Prior to this commit it was possible to create base_blobs with any arbitrary amount of bits, like base_blob<9>. One could assume that this would be a valid way to create a bit field that guarantees to have at least 9 bits. However, in such a case, base_blob would not behave as expected because the WIDTH is rounded down to the closest whole byte (simple integer division by 8). This commit makes sure that this oddity is detected and blocked by the compiler.

ACKs for top commit:
  MarcoFalke:
    lgtm ACK 5fc4939
  theStack:
    ACK 5fc4939
  stickies-v:
    ACK 5fc4939

Tree-SHA512: 6a06760f09d4a9e6f0b9338d4dddd4091f2ac59a843a443d9302959936d72c55f7cccd55a51ec3a5a799921f68be1b87968ef3c9c11d3389cbd369b5045bb50a

Author: fanquake

src/uint256.h

@@ -22,6 +22,7 @@ class base_blob
 {
 protected:
     static constexpr int WIDTH = BITS / 8;
+    static_assert(BITS % 8 == 0, "base_blob currently only supports whole bytes.");
     std::array<uint8_t, WIDTH> m_data;
     static_assert(WIDTH == sizeof(m_data), "Sanity check");