Function decode_length 

pub fn decode_length(mtype: MainType, length: u32, stype: SubType) -> u32

Decode header length field to actual bit length.

Inverse of encode_length. Returns the number of bits in the digest.

• META/SEMANTIC/CONTENT/DATA/INSTANCE/FLAKE: (length + 1) * 32
• ISCC + Wide: 256
• ISCC + other: popcount(length) * 64 + 128
• ID: length * 8 + 64