Bind with ... as targets to __enter__() return values

[BI71317]

Limitation of this PR

This PR addresses only the first item of #787 : binding the result of __enter__() to the as target.

Current state in pseudo code

The below code interpreted differently in each codon and python:

with Manager() as pair:
	body

In Codon

Lowering current with syntax in Codon behaves conceptually like this:

# assign Class Instance
pair = Manager()
# Only calls _enter_ method
pair.__enter__()
try:
    ...
finally:
    pair.__exit__()

In Codon, pair refers to the Manager Class instance in Codon, and the return value of __enter__() is discarded.

In Python

In a While, with syntax in Python behaves like below pseudo code:

# first  assign temporal Class instance
temp = Manager()
# assigns class methods
enter = temp.__enter__
exit = temp.__exit__
# Binding “pair” identifier with return value of enter method.
pair = enter()

try:
    ...
except:
    ...
finally:
    pair.__exit__()

https://docs.python.org/3/reference/compound_stmts.html#the-with-statement
In Python, pair refers to the return value of __enter__() .

What this PR fixes

Conceptually, lowering changes to:

_mgr = Manager()
var = _mgr.__enter__()
try:
    body
finally:
    _mgr.__exit__()

[BI71317]

And, yeah. It conflicts every other test cases with current with .. as .. syntaxes.

in codon\test\parser\typecheck\test_function.codon

#%% with,barebones
class Foo:
    i: int
    def __enter__(self: Foo):
        print '> foo! ' + str(self.i)
    def __exit__(self: Foo):
        print '< foo! ' + str(self.i)
    def foo(self: Foo):
        print 'woof'
class Bar:
    s: str
    def __enter__(self: Bar):
        print '> bar! ' + self.s
    def __exit__(self: Bar):
        print '< bar! ' + self.s
    def bar(self: Bar):
        print 'meow'
with Foo(0) as f:
#: > foo! 0
    f.foo()  #: woof
#: < foo! 0
with Foo(1) as f, Bar('s') as b:
#: > foo! 1
#: > bar! s
    f.foo()  #: woof
    b.bar()  #: meow
#: < bar! s
#: < foo! 1
with Foo(2), Bar('t') as q:
#: > foo! 2
#: > bar! t
    print 'eeh'  #: eeh
    q.bar()  #: meow
#: < bar! t
#: < foo! 2

[  FAILED  ] 9 tests, listed below:
[  FAILED  ] TypeTests/SeqTest.Run/parser_typecheck_test_function_codon_debug_198_with, where GetParam() = ("parser/typecheck/test_function.codon", true, "198_with", "#%% with,barebones\nclass Foo:\n    i: int\n    def __enter__(self: Foo):\n        print '> foo! ' + str(self.i)\n    def __exit__(self: Foo):\n        print '< foo! ' + str(self.i)\n    def foo(self: Foo):\n        print 'woof'\nclass Bar:\n    s: str\n    def __enter__(self: Bar):\n        print '> bar! ' + self.s\n    def __exit__(self: Bar):\n        print '< bar! ' + self.s\n    def bar(self: Bar):\n        print 'meow'\nwith Foo(0) as f:\n#: > foo! 0\n    f.foo()  #: woof\n#: < foo! 0\nwith Foo(1) as f, Bar('s') as b:\n#: > foo! 1\n#: > bar! s\n    f.foo()  #: woof\n    b.bar()  #: meow\n#: < bar! s\n#: < foo! 1\nwith Foo(2), Bar('t') as q:\n#: > foo! 2\n#: > bar! t\n    print 'eeh'  #: eeh\n    q.bar()  #: meow\n#: < bar! t\n#: < foo! 2\n\n\n", 161, true, false, true)