Only support FIELD_ELEMENTS_PER_BLOB values that are a power of 2. (#249)

* Intentionally fail to compile with a FIELD_ELEMENTS_PER_BLOB value that is not a power of 2.
We do not want to support or maintain this case, so let us be explicit about it.

* Update src/c_kzg_4844.h

Co-authored-by: Justin Traglia <95511699+jtraglia@users.noreply.github.com>

* Update src/c_kzg_4844.h

Co-authored-by: Justin Traglia <95511699+jtraglia@users.noreply.github.com>

---------

Co-authored-by: George Kadianakis <desnacked@riseup.net>
Co-authored-by: Justin Traglia <95511699+jtraglia@users.noreply.github.com>

src/c_kzg_4844.h

@@ -49,6 +49,18 @@ extern "C" {
 #error Invalid value of FIELD_ELEMENTS_PER_BLOB
 #endif // FIELD_ELEMENTS_PER_BLOB
 
+/**
+ * If FIELD_ELEMENTS_PER_BLOB is not a power of 2, the size of the FFT domain
+ * should be chosen as the the next-largest power of two and polynomials
+ * represented by their evaluations at a subset of the 2^i'th roots of unity.
+ * While the code in this library tries to take this into account,
+ * we do not need the case where FIELD_ELEMENTS_PER_BLOB is not a power of 2.
+ * As this case is neither maintained nor tested, we prefer to not support it.
+ */
+#if ((FIELD_ELEMENTS_PER_BLOB) & (FIELD_ELEMENTS_PER_BLOB)-1) != 0
+#error FIELD_ELEMENTS_PER_BLOB is not a power of two
+#endif
+
 #define BYTES_PER_COMMITMENT 48
 #define BYTES_PER_PROOF 48
 #define BYTES_PER_FIELD_ELEMENT 32