Make all useful data structures in Glow easy to dump in a textual form

[opti-mix]

Being able to dump any interesting data structures is very useful for interactive debugging and print debugging.

Currently, many of interesting data structures (e.g. glow::Value) have dump methods. Some others, e.g. glow::Node have getDebugDesc() method, but no dump method. And many interesting data structures don't have any method to dump their content, e.g. glow::Type.

Ideally, it would be nice to have the same uniform way to dump any data structure. LLVM usually uses dump for it.

It is also very useful to be able to get a string representation of the dumped content of any data structure as it allows for including it into a bigger composite output consisting of many different parts.

So, in short it would be nice to have e.g. the following methods for all interesting data structures in Glow:

• dump(llvm::raw_ostream &os)
• dump() which just calls dump(llvm::raw_ostream &os) with a default output stream as a parameter
• std::string toString(), which dumps into string

[superwizard2019]

@opti-mix
I've inserted those dump functions to glow::Node and glow::Type at #2969.
Could you list some other useful data structures that require dump functions?

[opti-mix]

Could you list some other useful data structures that require dump functions?

Add any missing helper methods (e.g. to produce a string or operator <<) for glow::Module, glow::Fucnction, glow::IRFunction, glow::Value, glow::Instruction and glow::Tensor. These are those most widely used data types in Glow.

In general it would be nice to also be able to dump/print all major runtime data-structures, e.g. ExecutionContext and the like. But all this probably would need to be implemented manually as C++ does not allow for introspection of types defined in the program (at least not without using a lot of macros/templates magic).