This way of doing nested parens was way easier than what I was trying
before.

This facilitates exploring PDF files that I can't actually parse yet;
I can still use the Pdf object to look at parts of the file.  For example:

    >>> d = pdf.read('../Descargas/dercuano.20191230.pdf')
    >>> d.trailer
    Traceback (most recent call last):
    ...
      File "/home/compu/izodparse/izodparse/pdf.py", line 202, in <lambda>
	parenstring.xform = lambda d: ('str', bytes(d[0][1])) # XXX croaks on anything with \()
    TypeError: 'tuple' object cannot be interpreted as an integer
    >>> d.get_indirect_obj(440)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/home/compu/izodparse/izodparse/pdf.py", line 290, in get_indirect_obj
	offset, plumb = result
    TypeError: cannot unpack non-iterable NoneType object
    >>> d.xrefs
    <izodparse.pdf.XrefSection object at 0x7feaef606a30>
    >>> d.xrefs[440]
    b'0000095363 00000 n\r\n'
    >>> d.xrefs.offset_of(440)
    95363
    >>> d.read(_)
    b'440 0 obj\r\n<< /Border [ 0 0 .1 ] /C [ .6 .6 1 ] /Contents (notes'

In this case there are two separate problems: I need to fix
paren-string parsing for the trailer, and I need to be able to read
fractions to read the .6.

Now it's izodparse.pdf.read instead of izodparse.pdftour.read_pdf.

I pointed it at a second file, and it immediately croaked on a
comment, so now it supports comments.  But now I need to properly
decode backslash-escapes and nested parens inside strings.

Not, sadly, object stream decompression, yet.

I renamed `parsecmaps.py` to `pdftour.py`.  Now it's possible to use
it to navigate the structure of at least one PDF file well enough to
parse a CMap out of it.

This involved adding some stream support, which involved tweaking the
parsing engine a bit.  In keeping with the rest of the
fast-and-loose-exploration nature of the program, it doesn't even
check for `endstream` after the end of the stream, much less `endobj`.

With that, and a bit of tweaking, my `cmaps_for_pages` code from last
week runs now!

Now I can open a PDF file and parse some objects out of it.  Soon I'll
be able to traverse the object graph.

Now we can see what grammar `csranges_to_grammar` has constructed for
us from the CMap file and whether it makes sense.