Better handling of Hashmaps

[Jazzpirate]

(builds on my other pull request for Box/Rc/Arc, but can be easily disentangled).

The blanket implementation for HashMaps has two too-narrow properties that can be generalized easily, as done by this pull requests:

• it disregarded the Hasher implementation in the generics for HashMap, i.e. it used HashMap<K,V> instead of HashMap<K,V,S>, which doesn't allow for using popular hashing crates such as fxhash and rustc_hash,
• it required that the keys implement From<Cow<'static, str>> - this PR generalizes this to TryFrom to allow for possibly fallible parsing of HashMap keys.

The latter could be changed to requiring serde_lite::Deserialize instead, with the implicit contract that the Intermediate used has to be a string, but that would a breaking change, whereas everything else in this PR should be backwards compatible.