Multiple muSE instances

When embedding muSE into an existing C/C++ program, there are a few ground rules for creating and using muSE environments a.k.a. execution contexts.

1. You are allowed to create multiple muSE environments in a single process.


2. You have to limit all expression evaluation to a single thread of execution.


3. You can switch between different environments using the muse_set_current_env() API call. If you don't the last created environment will be the current environment for evaluating expressions.


4. If you statically link muSE into a shared module (.dll/.so), each shared module gets its own current environment state and will not interfere with other shared libraries.

In the future, the current environment state may become thread-local, in which case you'll be able to use different environments simultaneously in different threads. Still, a single environment will be allowed to be the current environment of only one thread at a time. Note that the API won't need to change to support this behaviour extension.