Support Python-like class instantiation

[litlighilit]

To support:

class O:
  a: int
  def init(self, a):
    self.a = a
let o = O(3)

(currently the last line just must be written as let o = newO(3))

Gen O as macro/template

class O:... generates

type <genSym> = ref object
  <attr list>
template O*: untyped =
  bind <genSym>
  <genSym>
template O*(<init-list>): untyped =
    <wrap init func body>
<methods...>

(<xxx> is parameters to be instanced)

but there are some strictions, among which there is an unacceptable one:

Cannot support init with no params being called!
Because both O and O() calls the generated template.
So this cannot be adopted to class whose __init__ param-list is only self (e.g. all classes without a custom __init__)

Write generated type define into a new module (write to another file)

Nim disallows define a type and a proc with the same name in one module, but whe they're placed in separated modules, it's allowed. So we can write type O* = ref object ... to a new file(maybe tempfile) and add a import in current module.

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Such a tactic can also be used when implementing pylib, such as allowing writting str.maketrans instead of PyStr.maketrans