Add test code for stdlib

[arapower]

Added test code for std functions.

Unaddressed Items

• input function: Couldn't figure out how to write the code
  • Addressed
• trim_left function: Does not perform as expected
  • Addressed
• trim_right function: Does not perform as expected
  • Addressed
• exit function: Unable to validate changes in exit code.
  • Addressed
• Comprehensive test coverage: Only covered the happy path for now

I am not familiar with Rust, so please point out any problems.

Closes #117

[b1ek]

• input function: Couldn't figure out how to write the code

you could write code like print(input()) and feed something into stdin via rust, then check that stdout = stdin. i can't implement this today on my own as my workstation broke, but someone else could

[arapower]

@b1ek Can you take a look at this?

[arapower]

@b1ek Thank you for your commit.