Feature: require Java ≥ 7 to reduce code redundancy

[rillig]

Currently, the required Java version for sfntly is not documented anywhere. By experimenting I found out that with the exception of the SafeVarargs annotation, sfntly currently targets Java 6.

Changing the minimally supported Java version to 7 would bring these benefits:

• Less code to write for try-with-resources
• Less code to write for generic types
• Less code to write for implementing equals/hashCode

Changing the minimally supported Java version to 8 would bring these benefits:

• Even less code to write for generic types

The required Java version should be documented somewhere.