Add C-struct wrappers for imperative HMAC Cryptol model.
Add C-struct wrappers for imperative HMAC Cryptol model.

I.e., translate between the C-struct as it appears in the LLVM, and
the Cryptol record we use in the imperative HMAC model. They're almost
the same, except that the nested hash states are treated differently
(they're much more complex in the C-struct).

alexeblee/s2n,jldodds/s2n,gibson-compsci/s2n,bpdavidson/s2n,gibson-compsci/s2n,wcs1only/s2n,alexeblee/s2n,bpdavidson/s2n,awslabs/s2n,jldodds/s2n,bpdavidson/s2n,wcs1only/s2n,raycoll/s2n,PKRoma/s2n,alexeblee/s2n,PKRoma/s2n,PKRoma/s2n,awslabs/s2n,wcs1only/s2n,raycoll/s2n,PKRoma/s2n,awslabs/s2n,jldodds/s2n,awslabs/s2n,bpdavidson/s2n,bpdavidson/s2n,raycoll/s2n,gibson-compsci/s2n,gibson-compsci/s2n,wcs1only/s2n,jldodds/s2n,gibson-compsci/s2n,wcs1only/s2n,colmmacc/s2n,alexeblee/s2n,gibson-compsci/s2n,awslabs/s2n,wcs1only/s2n,PKRoma/s2n,PKRoma/s2n,PKRoma/s2n,wcs1only/s2n,wcs1only/s2n,alexeblee/s2n,colmmacc/s2n,raycoll/s2n,colmmacc/s2n,alexeblee/s2n,colmmacc/s2n,bpdavidson/s2n,awslabs/s2n,colmmacc/s2n,PKRoma/s2n,raycoll/s2n,colmmacc/s2n,jldodds/s2n,raycoll/s2n