[language/vm-runtime] make Interpreter private to the crate
[language/vm-runtime] make Interpreter private to the crate

This isn't used anywhere, nor should it be.

Closes: #2729
Approved by: dariorussi

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra