Use strong types for key and bare item integer, decimal, token, and string

[apasel422]

This is an updated version of #102 that takes the following approach:

1. BareItem and RefBareItem contain only fully validated types.
2. Dictionary and parameter keys are likewise fully validated.
3. A number of helpers based on Into and TryInto simplify creation of these types from unvalidated inputs, including at compile-time using const fn.
4. Parsing produces these fully validated types.
5. Serialization accepts only these fully validated types, making serialization in general infallible (the only real errors being empty list and empty dictionary) and presumably improving performance in the likely common case that components are fully validated at compile time.
6. rust_decimal::Decimal is replaced with an sfv::Decimal type that simply wraps the new Integer type, as an SFV decimal is 12 integral digits and 3 fractional digits, while an SFV integer is 15 decimal digits.
7. The new sfv::{Key, String, Token} types are the owned equivalents of the new sfv::{KeyRef, StringRef, TokenRef} types, analogous to Vec<T> vs [T].

As part of #119, we will likewise introduce an sfv::Date type that wraps Integer.

Fixes #98