Implement svg switch conditions in rust
Move the implementation of switch conditionals (requiredFeatures, requiredFeatures and systemLanguage attributes)to rust.
NOTE: these are my very first steps with rust, I tried to be as idiomatic as possible, but please point out any issue. I will comment on my own patch to point out a few things.
Apart from the rustification, the positive aspects are that the new code:
- supports space-separated requiredFeatures and Extensions as described in the spec
- has unit tests for all three attributes