Move Type1Font._tokens into a top-level function _tokenize that is a
coroutine. The parsing stage consuming the tokens can instruct the
tokenizer to return a binary token - this is necessary when decrypting
the CharStrings and Subrs arrays, since the preceding context determines
which parts of the data need to be decrypted.

The function now also parses the encrypted portion of the font file.

To support usage as a coroutine, move the whitespace filtering into the
function, since passing the information about binary tokens would not
easily work through a filter.

The function now returns tokens as subclasses of a new _Token class,
which carry the position and value of the token and can have
token-specific helper methods. The position data will be needed when
modifying the file, as the font is transformed or subsetted.

A new helper function _expression can be used to consume tokens that
form a balanced subexpression delimited by [] or {}. This helps fix a
bug in UniqueID removal: if the font includes PostScript code that
checks if the UniqueID is set in the current dictionary, the previous
code broke that code instead of removing the UniqueID definition. Fonts
can include UniqueID in the encrypted portion as well as the cleartext
one, and removal is now done in both portions.

Fix a bug related to font weight: the key is title-cased and not
lower-cased, so font.prop['weight'] should not exist.

Type-1 fonts are required to have subroutines with specific contents
but their names may vary. They are usually ND, NP and RD but names
like | and |- appear too.