Fix YAML::PullParser#read_raw

[ezrast]

Currently, there are a number of cases where read_raw returns a string that doesn't actually represent the content fed into the parser. This uses Builder to make sure that what comes out represents the same as what went in.

Example of bad behavior:
https://carc.in/#/r/2o8f

require "yaml"

struct Good
  YAML.mapping(
    foo: { type: String, nilable: true }
  )
end

struct Bad1
  YAML.mapping(
    foo: String?
  )
end

alias Bad2 = Hash(String, String | Int32)?

yamls = [
  %q(foo: " x "),
  %q(foo: "x #{y} z"),
  %q(foo: "- oops")
]

yamls.each do |yaml|
  pp Good.from_yaml yaml
  begin
    pp Bad1.from_yaml yaml
  rescue ex
    pp ex.message
  end
  begin
    pp Bad2.from_yaml yaml
  rescue ex
    pp ex.message
  end
  puts "-----"
end

[RX14]

@jwaldrip how does this PR interact with yours?