Raise useful error message when using a dynamic type as a key in a mapping

[bortzmeyer]

% solc multi-valued-mappings.sol   
Internal compiler error during compilation:
/build/cpp-ethereum-F1MTmh/cpp-ethereum-1.2.5~xenial/solidity/libsolidity/codegen/ExpressionCompiler.cpp(102): Throw in function void dev::solidity::ExpressionCompiler::appendStateVariableAccessor(const dev::solidity::VariableDeclaration&)
Dynamic exception type: boost::exception_detail::clone_impl<dev::solidity::InternalCompilerError>
std::exception::what: std::exception
[dev::tag_comment*] = Accessors for mapping with dynamically-sized keys not yet implemented.

% solc --version
solc, the solidity compiler commandline interface
Version: 0.3.3-0/RelWithDebInfo-Linux/g++/Interpreter

The source code is attached.
multi-valued-mappings.txt

[bortzmeyer]

Removing "public" from the variable "data" allow the program to be compiled. But I would prefer a clear error message than an internal error.

[axic]

@bortzmeyer the error message is there also:
[dev::tag_comment*] = Accessors for mapping with dynamically-sized keys not yet implemented.

[axic]

Duplicate of #498.

[Pipeliner]

@axic I guess @bortzmeyer meant that it was not immediately obvious from the error message that removing public would work. A more friendly message would probably be very helpful here.

[axic]

Actually it would be nice to raise this issue in the type checker so a proper error message can be displayed as opposed to an assertion.