Support input() function

[knogu]

It is important that Codon supports as many Python's features as possible.
But input() has yet to be implemented in Codon. So I made this PR to add it.
input() in Python: https://docs.python.org/3/library/functions.html#input

in develop branch (before merging this PR):

$ cat input.codon
print(input())
$ codon run input.codon
input.codon:1:7-12: error: name 'input' is not defined

in the branch for this PR, it works as well as Python
(test right under the command is manually input)

$ codon run input.codon
test
test

Closes #96

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