Fixed language server, improved cli and bugfixes

[alegnani]

• fixed a bug where the modulo operator would not be parsed in annotations
• added support for refute and assume statements
• implemented proper cli supporting input from stdin
• fixed @biw from being bits in word to bytes in word (fixes #9)
• partially implemented proper error handling
• automatically adds refute false to end of statements to check if proving false
• automatically adds heap length precondition to methods (uses cli argument for max heap size)
• implemented ViperHandle to reuse connection to the Viper server
• fixed language server to work with paths supplied by VS Code
• added custom command for current file verification