Bugfix: Prefix generated static variables to avoid reserved keyword clashing

[mcos]

I noticed earlier while playing around with this library that if a protobuf file namespace has reserved keywords, then a variables are generated with reserved keyword names. Importing the file then causes errors.

syntax = 'proto3';

package var.for.switch;

message Sampler {
	string sample = 1;
}

Compiling this and requiring it gives the following:

var var = {};
            ^^^
SyntaxError: Unexpected token var

To get around this, I simply prefixed the generated variable name with $, which seemed to be in keeping with the conventions of this library. These variables are generated within closures, so they are not assigned anywhere that they may be accessed by a consumer of the generated code.

I understand that this fix may be over-simplified, so I'm happy to work on a better solution than this. I also understand that there are no tests for this module yet, but if some were added, I'd be happy to contribute a test case for this.

[dcodeIO]

There are a few issues with this:

• While the declaration is renamed, subsequent uses are not (i.e. when returning from the closure or assigning properties).
• If a namespace is named protobuf, it will hide the global $protobuf var for example.

[dcodeIO]

Does this fix it?

[mcos]

Ah yeah, that was hacky of me, and I see now that $protobuf would be shadowed, which is really not good. Thanks for the fixes in be3e0d9 and 9b7b92a!