18c Fix: Logical operators

[ngjunsiang]

Oops, one bug that slipped testing!

Test code:

PROCEDURE TestBool(Succeeded : BOOLEAN)
    IF Succeeded = TRUE AND NOT (-1 = -1)
      THEN
        OUTPUT "Yay!"
      ELSE
        OUTPUT "Awww!"
    ENDIF
ENDPROCEDURE

CALL TestBool(TRUE)

This doesn't work, because ...
https://github.com/nyjc-computing/pseudo/blob/660d9fd53224954cf78282b4d79d06271ef7f4dc/parser.py#L88-L95

Our unary() parser is too generous, parsing an entire expression, which gives us -(1 = -1) instead of ((-1) = (-1)).

Simple fix: parse with value() instead of expression():
e4bdb4a

[ngjunsiang]

Another minor issue: errors involving names display a Name Expr instead of a token. That's because we didn't insert the correct token. Fix: [4420dc6]

And with that, we also wrap up two issues: #5 and #7.