Add ability to generate NEO3-compatable *.manifest.json into compiler.
This file represets contract manifest and includes ABI information, so
we don't need to create separate *.abi.json file. NEO3 debugger also
needs *.manifest.json only. So, switched from *.abi.json to
*.manifest.json file.
1. Slot is a new mechanism for storing variables during execution
which is more convenient than alt.stack. This commit implements
support for slot opcodes in both vm and compiler.
2. Remove old alt.stack opcodes.
3. Do not process globals at the start of every function, but instead
load them single time at main.
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.
