completed implementation of take() builtin

[hal0x2328]

First foray into trying to implement builtins into compiler, started with take() since it was already fleshed out in builtins.py. Test case seems to work:
`
neo> build boa/tests/src/TakeTest.py test 05 05 False
Saved output to boa/tests/src/TakeTest.avm

---

Calling boa/tests/src/TakeTest.py with arguments []
Test deploy invoke successful
Used total of 38 operations
Result b'hello'
Invoke TX gas cost: 0.001
`

[coveralls]

Coverage remained the same at 50.954% when pulling d8d4206 on hal0x2328:feature-boa into 2bcdcca on CityOfZion:feature-boa.

[localhuman]

Looks good, though I'll have to test is out when I get home from work.

The same functionality can be done with slice notation, so currently str[:5] should return the same thing.