Rethink the top-level unrolled block and record APIs

[woodruffw]

llvm_mapper::unroll::UnrolledBlock currently has a bunch of APIs with signatures that look roughly like this:

pub fn one_record(&self, code: u64) -> Result<&UnrolledRecord, BlockMapError> { ... }
pub fn one_block(&self, id: BlockId) -> Result<&UnrolledBlock, BlockMapError> { ... }

These are convenient in each record/block mapping implementation, but they're also constraining: because they return BlockMapError as their error type, downstream types need to either accept that type (e.g. with thiserror's #[from]) or mangle it into something more local and context sensitive. This in turn makes it harder to make the mapping traits more flexible in terms of error handling.

[woodruffw]

20.