Support named unicode characters in f-strings

[thatch]

Fixes #154

The previous behavior misinterpreted the curly braces as enclosing an
expression. This change does some cursory validation so we can still
get parse errors in the most egregious cases, but does not validate that
the names are actually valid, only that they are name-shaped and have a
chance of being valid.

The character names appear to obey a few rules:

• Case insensitive
• Name characters are [A-Z0-9 \-]
• Whitespace before or after is not allowed
• Whitespace in the middle may only be a single space between words
• Dashes may occur at the start or middle of a word

f"\N{A B}"           # might be legal
f"\N{a b}"           # equivalent to above
f"\N{A     B}"       # no way
f"\N{    A B     }"  # no way
f"""\N{A
B}"""                # no way

For confirming this regex matches all (current) unicode character names:

import re
import sys
import unicodedata

R = re.compile(r"[A-Za-z0-9\-]+(?: [A-Za-z0-9\-]+)*")

for i in range(sys.maxunicode):
    try:
        name = unicodedata.name(chr(i))
    except ValueError:
        # Some small values like 0 and 1 have no name, /shrug
        continue
    m = R.fullmatch(name)
    if m is None:
        print("FAIL", repr(name))

[isidentical]

LGTM. Could you please add a few tests where these unicode escapes used in the middle and at the end of the f-string. Like; f"some {stuff} and \N{escape}" etc.

[thatch]

Additional tests in latest push.

[isidentical]

Thanks for the patch @thatch! Also loved your work on https://github.com/thatch/python-grammar-changes