removed depracation warnings and substituted

[FerrisWasTaken]

In Detail

• 1. Removed impl for describe method or meval::error::Error
• 2. changed try!() to ? operator
• 3. changed ... 's in tokenizer.rs to ..= 's in line 129 and 124
• 4. removed redundant import of nom::Needed
• 5. converted un-generated doc to comment line 148
• 6. changed mentions of traits to dyn trait

[rekka]

Thank you for the pull request. I currently am a bit short on time so it might take me a week or two to review this.