Make env_logger optional

[wks]

Currently, mmtk-core includes a log backend, that is, env_logger. It also attempts to configure the env_logger in memory_manager::mmtk_init. This approach, although convenient, may have a problem when the VM wants to include its own logging settings for various reasons. For example,

1. The VM binding also uses logs, and wants to log messages before mmtk_init.
2. The VM has some parts written in Rust, too, and they also need logging. (Ruby's YJIT is written in Rust.)

We can have env_logger guarded behind a Cargo feature, or the other way around -- adding a Cargo feature to not include env_logger as part of our dependencies.

[qinsoon]

Sounds good. This could also be a runtime option.

Currently we create env_logger in mmtk_init, and we allow that to fail. That means the binding can initialize a Rust logger before calling mmtk_init