Extend the coverage of UsesTypeParams and UsesLifetimes

Add the usage module, with UsesTypeParams and UsesLifetimes. These traits help track usage of generic parameters throughout a proc-macro input, simplifying bounding or generation of derivative structs. See #40 and #41

• Fixed an issue with supports(struct_*) caused by the syn upgrade, and added tests to make sure it doesn't break again.

Rename FromMetaItem to FromMeta

• Add ast::Generics and ast::GenericParam to work with generics in a manner similar to ast::Data
• Add ast::GenericParamExt to support alternate representations of generic parameters
• Add util::WithOriginal to get a parsed representation and syn's own struct for a syntax block
• Add FromGenerics and FromGenericParam traits (without derive support)
• Change generated code for generics magic field to invoke FromGenerics trait during parsing
• Add FromTypeParam trait #30. Thanks to @upsuper

• Update dependencies #26. Thanks to @hcpl

Add support for proc-macro-2 (#24)

Breaking Changes

• Update syn to 0.12 #20. Thanks to @Eijebong
• Update quote to 0.4 #20. Thanks to @Eijebong
• Rename magic field body in derived FromDeriveInput structs to data to stay in sync with syn
• Rename magic field data in derived FromVariant structs to fields to stay in sync with syn

• Update lazy_static to 1.0 #16 - thanks to @Eijebong