Implemented getenv() and setenv() in the 'system' module

getenv() and setenv() basically work like the C99/POSIX versions,
except our setenv() will also delete a variable if the value
argument is nil.

Added a test script envtest.eel that'll show some environment
variables, and test the basic operations on a test variable.

NOTE: The envtest.eel uses system() to run an echo command. I
suspect the Windows console won't understand the syntax...

Un*x and Win32 implementations, though the latter is untested.

Closes #5.