Refactor indented blocks

[Gauntlet173]

The original indented blocks version of the parser was discarded in favour of pre-processing the text and converting indents and undents into symbols, because of errors reported at pyparsing/pyparsing#369 (comment). It looks like fixes have been made to pyparsing in >=3.0.7 that might make it possible to re-implement the parser using the indented_block parser class, which would potentially clean up the code a bit.