Change File and FileManager macro guards prefix according to specified namespace

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The --namespace argument was designed to allow users to include the generated files within their own custom namespace instead of using hardcoded namespace bin2cpp.

When users want to generate files in two different custom namespaces, the File and FileManager interfaces are guarded by the same macro guards. Specifically by the macro guards BIN2CPP_EMBEDDEDFILE_CLASS and BIN2CPP_FILEMANAGER_CLASS which is a problem. Each namespace should have an independent (but unfortunately identical) File and FileManager interfaces.

The issue is that when a source file includes two headers of each namespace, the second header cannot register the interfaces because the other header has already defined the macro guards.

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The solution is to change the prefix of the macro guards to match the custom namespace. For example, if using the namespace foo, the macro guards should be generated as FOO_EMBEDDEDFILE_CLASS and FOO_FILEMANAGER_CLASS.