Create a proc-macro in sigma-rust that takes ergotree from scala side (e.g. https://github.com/ScorexFoundation/sigmastate-interpreter/blob/d254cad473c974889981b03ac2368464b0ec200c/sigmastate/src/test/scala/special/sigma/SigmaDslSpecification.scala#L243-L249) and generates an equivalent instance of ergotree_ir::mir::expr::Expr.

We need this because it is too laborious to manually craft ergotree_ir::mir::expr::Expr instances. There are about 180 tests in the scala JIT test suite. Copy-pasting these ergotrees is essential.

Part of #193