Investigate better types for max len of bounded storage items

[emostov]

Currently, bounded storage items require a type that implements Get for its max length. This permits the user to configure the type as something that can be modified at runtime.

Modifying the bounds at runtime is a foot gun for chain builders. For example, it could render some storage items non-decodable or break assumptions used to create weights and soften DDoS vectors.

We should look into forcing the bound to be a const or something similiar

[kianenigma]

Generally speaking, I realized again that Get can break everything.

For example, if you put a type Foo: Get<u32> into the metadata, but the Get<_> is actually implemented by a real storage that can change on the fly, PJS api breaks.

[juangirini]

related issue #244