Reduce memory footprint by reducing size of data types

[schungx]

This PR attempts to reduce the memory footprint of using this crate.

• String types are changed to Box<str> to save 8 bytes each.
• The thin-vec crate is used to replace Vec's, saving 16 bytes each.
• Many large Option's are Boxed
• Many large types in HashMaps are Boxed

I try to avoid making code-breaking changes, but some return types are changed, such as &Vec<T> to &[T]... in most cases that shouldn't matter. The entire test suite compiles are run without change.

The current API design prevents more involved optimizations without breaking code. For example, returning &bool, &u32 etc. prevents those fields from being replaced by smaller, packed data types like bitflags. A future major version should really change those to return non-references instead.

I have given up on optimizing chart data types because they are simply too tedious, and there shouldn't really be more than a few charts in a typical spreadsheet.

[MathNya]

@schungx
Thank you for the tremendous corrections.
We have no problem with the revisions and will approve them.

[schungx]

Beware some function signatures have minor subtle differences which may require code changes. Be prepared for some incoming issues on this.

[schungx]

When I have time I'll put in #[inline] on all the trivial APIs.

[MathNya]

Noted.
We will include that in the release notes.