Faster NameAnalyser implementation

[marcoeilers]

The current NameAnalyser's check to ensure that local variables are not used before they are declared is really really slow for large methods (see issue #630).
It first walks through the entire PAST and collects all declarations, then, in a second pass, it iterates over the the entire PAST again and, for every local variable reference it finds, walks through all AST nodes occurring in the current method again and checks if the current variable reference is found before its declaration (method containsSubnodeBefore).

This PR simplifies the process as follows:

• We first walk over all global member declarations to collect all globally available names.
• Then we walk through each member completely a single time, collecting all local declarations in the process, and checking for all references if they are already defined in the current context (which contains all global declarations, as well as all local declarations we have found so far).

This has the drawback that our error message can no longer state whether we are referencing a local variable that has not been declared yet or one that has not been declared at all, but it massively speeds up the entire checking process for large methods.

Also, I added an isPure function on the AST node for Viper functions because I need this for something else.

[fpoli]

Nice, thanks! I tested on the programs of #630 and Silicon terminates in a few seconds now.