A part of integration with NEO Blockchain Toolkit (see #902). To be
able to deploy smart-contract compiled with neo-go compiler via NEO
Express, we have to generate additional .abi.json file. This file
contains the following information:
- hash of the compiled contract
- smart-contract metadata (title, description, version, author,
email, has-storage, has-dynamic-invoke, is-payable)
- smart-contract entry point
- functions
- events
However, this .abi.json file is slightly different from the one,
described in manifest.go, so we have to add auxilaury stractures for
json marshalling. The .abi.json format used by NEO-Express is described
[here](https://github.com/neo-project/neo-devpack-dotnet/blob/master/src/Neo.Compiler.MSIL/FuncExport.cs#L66).
Method `methodInfoFromScope(...)` always returned an empty parameters
set, so we were missing this information in both .abi.json and
.debug.json files. Fixed now.
Sequence points is a way to map a specific instruction offset
from a compiled contract to a text span in a source file.
This commit implements mapping only for `return` statements.
Further improvements are straight-forward.
