Mini-RFC is here: https://discuss.tvm.apache.org/t/mini-rfc-name-mangling-in-aot

With this change we'll mangle the name of global symbols so that we can bundle
together multiple models in the same application.

The relay.build interface has been left unchanged, which means I am
resuing mod_name as a prefix for all functions. If mod_name is None then
a "_tvm" prefix is used.

I had to add two different compilation functions:
- _CompileEngineLowerWithModuleName to mangle all the operators with the mod_name
- PartitionGraphWithModName to mangle all the operators produced by BYOC

I could have changed signature of both, but that would have meant a very
invasive refactoring.

I refactored the aot test utils and added some tests for multiple
models.

Change-Id: I30e93fa075f660054577ea36cf9268ec0c6eebcb

Change-Id: I4f11da7fce1327ad89bb25f25209b57077b2c6a3