[ fix #326 ] Interleaving let binding and declarations

[gallais]

This is a work in progress. Compilation currently fails with the
following internal error:

idris: Erasure.hs:lamToLet': unexpected input: vs = [{eta_4}], tm = Idris.Parser.case block in case block in mkLets at ./Idris/Parser.idr:602:8-19 at ./Idris/Parser.idr:602:22-26 {e_0} {e_5} {e_6} {e_2} !!V 0!! !!V 0!!
CallStack (from HasCallStack):
error, called at src/Idris/Erasure.hs:617:27 in idris-1.3.2-0f546d8533795a7031ff67b47abc3f394a048b391a392808377322d50f6ed1f3:Idris.Erasure

[ziman]

This seems to happen in overapplied lambdas, which I did not address. The RHS in Erasure.hs:617 should probably be mkApp tm vs.

[ziman]

mkApp tm vs typechecks but the de Bruijn indices in vs will probably be wrong because we're reapplying them under new let bindings. The existing code of lamToLet probably suffers from this bug already. :(

This will need a bit more work.