Fix #74 - prevent borrowing 'static from non-'static

[finnbear]

Fixes #74

This bug only affects you if you have a &'static _ field, such as &'static [u8] or &'static str, and you decode from a non-'static buffer. No other lifetimes are affected.

• Adds a '__de: 'static bound for impl Decoder and impl Decode of any item with a &'static _ field, except PhantomData<&'static _>.
  • This skips impl Default and impl View, but I didn't think adding complexity was justified for no change in which user programs compile
• Test that the bug no longer compiles
• Test that it is still possible to decode a struct field of type &'static str from a 'static buffer
• Test that it is still possible to decode a struct field of type PhantomData<&'static str> from a non-'static buffer
• Test that it is still not possible to decode a struct field of type StaticStr (where type StaticStr = &'static str), even from a 'static buffer

Diff for a simple struct with one 'static field: