Slightly better parse errors

[marcoeilers]

• Extracting only the relevant information from FastParse (i.e., what was expected at the point where parsing fails), instead of the whole (?) rule tree
• Renaming some parse rules, since they can be shown in the error message
• Removing an "Expected" that was in the message twice.

Before:

  [0] Parse error: Expected Expected entireProgram:1:1 / programDecl:4:1 / ("(" ~ exp.rep(0) ~ ")" | ":=" | "," | ";" | stmt | "}"):473:29, found "(vwa cd,nu" (test.vpr@473.29)

After:

  [0] Parse error: Expected ("(" ~ exp.rep(0) ~ ")" | ":=" | "," | ";" | stmt | "}"):473:29, found "(vwa cd,nu" (test.vpr@473.29)