IN, OUT and ATAN2

Added IN function.
Added OUT statement.
Added ATAN2 function.

case insensitive variables

Variables were case-sensitive because the interpreter stored and looked them up using the raw spelling from the source. I changed that boundary so names are normalized to uppercase on every write and read path in interpreter.go, which makes X, x, and x all resolve to the same variable.

RND fixed

Got rid of the float constant used for testing and changed it to rand.Float64

Keyword fix

Implicit LET was causing FOR LOOP type keywords to fail.

stability test

Change module path to github.com/misterunix/tinybasic

oppps

Testing 0.0.1-alpha

First run at making this a working module.