Update the parser for syn-0.15

[lukaslueg]

Our parser for macro_rules!()-syntax uses syn, which has recently bumped to 0.15, bringing a lot of changes. Rewrite the parser for syn-0.15.

The current parser for macro_rules!()-bodies is using syn-0.14. It does have some unresolved issues (1,2,3) which need fixing anyway.

One can use the existing tests (cargo test) as the last line of defense - those should parse in any case. A more comprehensive test is accessed via cargo test -- --ignored, which parses, builds and verifies a large number of macros. One can also use cargo run --example various to quickly generate a large number of examples.

[lukaslueg]

closed via #19