Generate docs in Construct compiler

[Mimickal]

This PR populates the name and docs attribute for top-level Construct Structs, and the docs attribute for fields. Since Construct is Python-based, I've pulled the now-common logic for Python out to a utility.

Example ksy:

meta:
  id: docs
  endian: le
doc: |
  Example to show off Construct doc strings

  These can be multiline
  and have lots of info and stuff.
doc-ref: https://construct.readthedocs.io/en/latest/advanced.html?highlight=description#documenting-fields
seq:
  - id: field1
    type: u4
    doc: 'Short desc'
  - id: field2
    type: other
    doc: This is a cool field
    doc-ref: https://www.google.com
  - id: field3
    type: u1
types:
  other:
    seq:
      - id: sub1
        type: u4

Before this PR:

from construct import *
from construct.lib import *

docs__other = Struct(
	'sub1' / Int32ul,
)

docs = Struct(
	'field1' / Int32ul,
	'field2' / LazyBound(lambda: docs__other),
	'field3' / Int8ub,
)

_schema = docs

After this PR:

from construct import *
from construct.lib import *

docs__other = 'docs__other' / Struct(
	'sub1' / Int32ul,
)

docs = 'docs' / Struct(
	'field1' / Int32ul * 'Short desc.',
	'field2' / LazyBound(lambda: docs__other) * \
		'''This is a cool field.
		
		.. seealso::
		   Source - https://www.google.com
		''',
	'field3' / Int8ub,
) * '''Example to show off Construct doc strings

These can be multiline
and have lots of info and stuff.

.. seealso::
   Source - https://construct.readthedocs.io/en/latest/advanced.html?highlight=description#documenting-fields
'''

_schema = docs

In Python interpreter:

>>> from docs import docs
>>> docs.name
'docs'
>>> docs.docs
'Example to show off Construct doc strings\n\nThese can be multiline\nand have lots of info and stuff.\n\n.. seealso::\n   Source - https://construct.readthedocs.io/en/latest/advanced.html?highlight=description#documenting-fields\n'
>>> docs.field2.docs
'This is a cool field.\n\t\t\n\t\t.. seealso::\n\t\t   Source - https://www.google.com\n\t\t'