Clairify definition of Identity Merkle Root

[roconnor-blockstream]

As noted by @uncomputable, the definition of the IMR in the Tech Report is not the same as the definition used in Haskell and C code. In the Haskell and C code the Tech Report defined IMR is additionally hashed with the input and output types. This is done to implement the Tech Report requirement that ⟨imr, input type, output type⟩ triples are unique (up to finding hash collisions). However the resulting terminology is confusing.