Trait bounds: derive as much as is reasonable, `#[scale_info(bounds())]` for the rest

[ascjones]

In a meeting it was decided that we put forward with scale-info in the following way:

• We will go with the "hacks" that have already been implemented for it and for parity-scale-codec crates to cover the most common use cases with 80/20 rule.
• For the other 20% of cases that are not covered by the scale-info and parity-scale-codec derive macros we will provide a feature for users to define custom trait bounds similar to the design serde chose. Those custom trait bounds will be replacing the generated ones instead of being additive.
• We might consider in the future to remove the "cruft" of "hacks" for common cases and consider removing them altogether with a major release for both derives.

Originally posted by @Robbepop in #76 (comment)

[dvdplm]

cc @thiolliere

[ascjones]

Linking paritytech/parity-scale-codec#263