bug: Panic when let statement is not correctly used

[Mido-sys]

Description

When an uninitialized variable is called plush panics. Due to parseLetStatement() returns nil if the next token isn't =. As a result, when parseProgram() calls stmt.String() it panics as stmt is holding a nil value.

To Reproduce

let foo
if (foo){
 foo = 1
}

[Mido-sys]

@sio4 I sent a PR request

[sio4]

(Note for future reference: The description is almost correct but the condition is not "when an uninitialized variable is called" but "the let statement is not correctly used without equal sign".)

[sio4]

I reverted merged PR #171 with #172. Please take a look at the comments on PR #171 and please consider filing a new PR! I will do my best to review that this time! (Sorry again for the delayed review :-)

[Mido-sys]

@sio4, I sent another PR :)