Use subscripting for generics.

The angle brackets syntax was originally suggested by @hynek and @glyph (IIRC), I had been using curly braces origianlly.

However a review of [Andreas Stefik and Susanna Siebert. 2013] suggests that subscripting is more readily understood by non-programmers.

(Full citation: Andreas Stefik and Susanna Siebert. 2013. An Empirical Investigation into Programming Language Syntax. ACM Transactions on Computing Education 13, 4, Article 19 (November 2013), 40 pages.)
Merge pull request #51 from alex/switch-generics-syntax

Use subscripting for generics.