Make BlockReader abstract, with varying impls.

This allows cool things to be accomplished through chaining BlockReaders
together without having to handle the block plumbing yourself.

The current set of implementations are a simple one that reads blocks
from a scanner delimited by a pattern, one that has a queue of blocks it
will attempt to pull from before reading, and one that triggers an
action before a block is read.

As an example use, for FDS, a combo of simple -> triggered -> pushback
is used where the triggered is used for prompting the user, and the
pushback supports macros.

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