Reset all per-parse state in JSON::ResumableParser#clear

[mame]

Currently, #clear does not reset current_nesting.

require "json"
parser = JSON::ResumableParser.new(max_nesting: 10)
10.times do
  parser << "[1"
  parser.parse
  parser.clear
end
parser << "[1"
parser.parse   # JSON::NestingError: nesting of 11 is too deep

This PR resets current_nesting.
It also resets in_array and emitted_deprecations just for case, though I am not sure whether we should reset emitted_deprecations or not.

[byroot]

I wonder if we should just memset(parser, 0, sizeof(*parser)).

[byroot]

I guess not. Because then we need to be careful not to leak the buffers we allocated etc. Nevermind.