Rewrite plistlib with functional style

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-12-01.19:28:36.864>
labels = ['3.7', 'library', 'performance']
title = 'Rewrite plistlib with functional style'
updated_at = <Date 2017-12-26.10:47:13.731>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2017-12-26.10:47:13.731>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-12-01.19:28:36.864>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 32196
keywords = ['patch']
message_count = 3.0
messages = ['307404', '307508', '309058']
nosy_count = 2.0
nosy_names = ['ronaldoussoren', 'serhiy.storchaka']
pr_nums = ['4671']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue32196'
versions = ['Python 3.7']