Clean up parsing code

[axch]

Enabled by (or noticed during?) f5683f157b4193be63e84a237f919c2889cda7da .

• [x] The whole split_program pattern is redundant, because it cycles through the parser's ability to parse multi-instruction streams anyway.
• [ ] split_instruction in the parsers should be pretty useless; flush it?
• [ ] expression_index_to_text_index in the parsers is looking pretty useless; flush it?

[axch]

Entry points into parsing:

• Console calls ripl.execute_instruction or ripl.execute_instructions
• ripl.execute_instructions calls
  • p.split_program
  • p.parse_instruction
  • then ripl.execute_parsed_instruction
• ripl.execute_instruction calls
  • p.parse_instruction or
  • ripl._ensure_parsed
  • then ripl.execute_parsed_instruction
• ripl.execute_program{,_from_file} calls
  • ripl.parse_program which
    • does the horrible thing with the comments
    • calls ripl.split_program
    • calls ripl._ensure_parsed
  • ripl.execute_parsed_program
    • which special-cases "load" instructions!?
• ripl._ensure_parsed bottoms out in
  • p.parse_expression
  • p.parse_number

The error reporting also interacts with the parser

• split_instruction is used for annotation (after the fact)

All of which bottom out in parse_instructions (with various manipulations of the returned location dicts).

[riastradh-probcomp]

Please flush all the splitting crud!

[axch]

Do I detect evidence of a splitting headache?

[riastradh-probcomp]

Yes -- standard remedy is to drink lots of water to flush it out.