Merge DoFastInvoke and DoInvoke in the interpreter.

The methods are almost exact copy-paste of each other.
Merge them to make modifications and maintenance easier.

I added template argument to somewhat keep the current
semantics, but I might try to remove the argument later.

Test: test.py --interpreter --host
Change-Id: I164cee8084857e9ae4f16939b3b2a836f049e83c