Now the built-in env_logger is guarded behind a Cargo feature
"builtin_env_logger".  It is a default feature, but can be disabled
in Cargo.toml by setting `dependencies.mmtk.default-features = false`.
In this way, VM bindings that want to implement its own logger can
remove the env_logger crate from its dependencies.

memory_manager::mmtk_init simply calls `try_init()` now.