This reverts commit b7e1734.

…tra memchr searching the remaining characters when no more backslashes exist.

Co-Authored-By: Jean Boussier <[email protected]>

A plain `char` may be `signed` or `unsigned` depending on the
implementation.  Also, bitwise ORing of `signed` values ​​is not
guaranteed to be `signed`.  To ensure portability, should logical-OR
each comparison, but casting to `signed char` is usually sufficient.

Fix non-portable code

…tially uninitialized

…n copy_remaining_bytes to avoid a branch to MEMCPY. Additionally use a space as padding byte instead of an 'X' so it can be represented diretly on AArch64 with a single instruction.

Fix: #929

When calling `cState_partial_generate` from `mHash_to_json` or other
`to_json` funcs, `VState` becomes unreachable very quickly, hence the
compiler may optimize it out of the stack, and make it invisible to
the GC stack scanning. This is particularly liekly given how aggressively
we inline.

Repro:

```ruby
require 'json'

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

10.times do
  test_data.to_json
end
```

But in practice the cause was just that the issued warning calls
Hash#inspect on a big hash, which triggers GC.

So it can be triggered even more reliably with:

```ruby
require 'json'

module JSON
  module Common
    def self.on_mixed_keys_hash(...)
      GC.start
    end
  end
end

test_data = {
  "flag" => true,
  "data" => 10000.times.map { [1.0] },
  :flag => false,
}

test_data.to_json
```