Fix "environment defunct error"

In some cases, people are presented with the NULL environment defunct error.
This appeared to be because of the way environments are passed around between testwhat functions.
After realizing that environments are actually not necessary in testwhat functions, as everything is standalone through the student_env and solution_env, all these arguments were removed from the code.

Improve documentation

Some improvements to the documentation.

Fix test_object, improve test_correct

• test_object() first checked if the classes correspond, then checks if the the values are ok. In some cases, this can cause issues with coercing (numeric and integer, for example). This has been wrapped in a test_correct() like structure now.
• test_correct() made use of test_or() behind the scenes, but the latter is less efficient. test_correct() no longer uses test_or() now.

Overall improvements and added functionality

• In test_function(), you can set incorrect_msg to a vector of messages, corresponding to the arguments you want to check. That way, your custom feedback is argument-specific
• test_expression_result() has been improved when the eq_conditoin = "equal"
• You can now specify a count of hits for test_student_typed().
• Added test_predefined_objects(), to check whether predefined objects are still ok.
• Added test_output_regex(), to check whether the output contains a certain pattern (regex or exact matching are possible)
• test_what() function cleaned up; you can only specify the feedback argument. This should be a character string or a list, containing at least a character string named message.