experiment(journal): Add some TH to compute offsets

[symbiont-daniel-gustafsson]

This is an experiment to automate the computation for offset of fields in a
record. This computation doesn't take alignment into account (and I'm not sure
it should). A benefit of having the macro is that when the type changes we don't
have to manually change the constants.

It works by adding a new macro offsetTo that given a field name computes the
size of all fields preceeding it.

*Journal> :set -XTemplateHaskell
*Journal> import OffsetMacro
*Journal OffsetMacro> $(offsetTo 'mdTermLength)
32
*Journal OffsetMacro> lOG_TERM_LENGTH_OFFSET
32

At the moment it computes to an expression that will do the sum at runtime, this
probably will be optimized away by the compiler.

This do require Template Haskell to work, with all what that entails, so not
sure yet if it is worth it.

[symbiont-stevan-andjelkovic]

One way around the problems of TH could perhaps be to only use this TH code in the testsuite to assure that the constants are correct, like a contract test?

I'm thinking something like:

data D = D
  { f1 :: A1
  , ...
  , fn :: AN
  }

d_F1_OFFSET = 0
d_FN_OFFSET = sizeOf (undefined :: A1) + ... + sizeOf (undefined :: AN)

-- In the testsuite:
assertOffsets 'D [d_F1_OFFSET, ..., d_FN_OFFSET]

This should catch if types of the fields change, if fields get renamed, if fields get removed or added (or rearranged). Is there anything else we need to worry about?