gensym vars now use X__gensymY instead of X`gensymY

[timotheecour]

this PR replaces backtick by __ (double underscore) in gensym symbols.

rationale:

• simpler
• ` is confusing, eg in js it causes this code:

import std/jsconsole
template fn() =
  var x0 = 2
  echo astToStr(x0)
  console.log(x0)
fn()

to escape the backtick via HEX60 which obfuscates the code for no reason:
x0HEX60gensym0_704643076

• it's hard to backtick X`gensymY (eg in docs etc) because it contains a backtick

• double underscore is already illegal in nim so there is no risk of accidental conflation

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. If you think it is still a valid PR, please rebase it on the latest devel; otherwise it will be closed. Thank you for your contributions.

[ringabout]

It doesn't work.