Refactor `Shape`

[nrc]

Shape is one of the key abstractions in Rustfmt, representing the space into which an AST node can be written. Unfortunately it is a mess - the structure is weird, the fields are badly named, and the methods are unintuitive. Documentation is also non-existent. We can do better!

[nrc]

Some thoughts on how to do it:

• we should not have so many different kinds of indent - backing out of an indent shouldn't be the shapes responsibility. We might want to have something like a stack of shapes or something to make that easier
• we should store an rhs offset and compute the width, rather than storing the width
• we might want to get rid of Indent and just have Shape, although Indent does abstract over tabs/spaces nicely
• ideally we should not force clients to manually adjust the shape, but when we should have a unifed mechanism to write into a buffer and compute the 'next' shape