support using decode routines from the full DR library without initialization

This issue is about having DR's decode routines automatically invoke standalone init if called prior to DR init.

This use case is what's driving this issue: creating a library that uses drdecode only, but that is able to be used both in a native app and in an app running under full DR. This does not work because drdecode relies on replacing heap_alloc with a version that calls malloc.

The initial workaround is to link the library with full DR, but that doesn't work unless the library forces all linking apps that use it but don't themselves run under DR to invoke dr_standalone_init().

Having automated standalone init will also make it easier for other standalone uses of DR.