Restructure Ast module

[shonfeder]

This aims to improve the organization of the Ast module. The key changes:

• Avoid duplication of types by use of module include type of
• Break up Inline types and Block types into seperate modules
• Improve naming on block type funtors
• Correct name of Block module to Block_parser, cause it's a parser, not a representation of blocks

This was spun out of work on #288 and is a pre-requisite for the changes there, as it allows sharing the high-level doc type.

[shonfeder]

Everything here is working quite nicely, except that the documentation generated is mangled a bit:

I'm trying to figure out how to fix that.

[shonfeder]

Thanks for looking!