Better errors for bad indents

[eemeli]

This replaces the single error "Document is not valid YAML (bad indentation?)" with three more specific ones:

• "All collection items must start at the same column"
• "A collection cannot be both a mapping and a sequence"
• "Document contains trailing content not separated by a ... or --- line"

I've also recently added the following related errors:

• "Block scalars must not be less indented than their first line"
• "Block scalars must not be less indented than their explicit indentation indicator"
• "Block scalars with more-indented leading empty lines must use an explicit indentation indicator"

Given all that, the CST parser should now far more rarely produce documents with more than one content node, and the AST doc.parse(cst) method could be modified to always set doc.contents to the first content node's value, rather than including all of them as an array in some cases.

These changes mean that invalid input is handled a little bit differently than before, including the CST that's produced from it. Where possible, the graph should now more closely resemble its intended form, rather than just breaking in two.