Refactor main module to support multiple backends

[manuranga]

Refactor compile.bal such that import resolution and job system can be shared by multiple backends.

Prerequisite for #1031

[manuranga]

I propose a new object type to be implemented per each backend. Implementation classes will live in separate files in the main module. (Moving it into respective backends will create a dependency between nback and llvm.print)

proposed interface:

type Backend object {
   // Build and write the output to a file.
   function buildModule(bir:Module birMod) returns CompileError;
   // Called after all calls to buildModule.
   function buildInitModule(map<bir:FunctionSignature> potentialEntryFuncs) returns CompileError;
}

[jclark]

That looks reasonable, but I don't think we need separate files.

[manuranga]

I was thinking of Emitter or EmitStage and emitModule.
https://en.wikipedia.org/wiki/Emitter