Make the repr of lambda contain signature and body expression.

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-09-30.18:14:23.113>
labels = ['interpreter-core', 'type-feature', '3.8']
title = 'Make the repr of lambda contain signature and body expression.'
updated_at = <Date 2018-10-22.19:26:54.089>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2018-10-22.19:26:54.089>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2018-09-30.18:14:23.113>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 34856
keywords = ['patch']
message_count = 8.0
messages = ['326738', '326747', '327179', '327181', '327183', '327202', '327205', '328271']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'rhettinger', 'terry.reedy', 'serhiy.storchaka', 'xtreak']
pr_nums = ['9647']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue34856'
versions = ['Python 3.8']