It used to be compiled to the static string in the compiler. But the
encoding status actually depends on the runtime configuration. A new
method `Kernel#__ENCODING__` is introduced to implement the feature.

Otherwise, the following code will crash:

```ruby
1.times{{}until 1; break}
```

Rename `MRB_WORDBOX_USE_HEAP_FLOAT` to `MRB_WORDBOX_NO_FLOAT_TRUNCATE'.

This reverts commit 913a0a5a.
In some cases, `OP_HASHADD` operand may not be `Hash`. We should check
explicitly in those cases.

In addition, add `NODE_SYM` for always true expression.

The invocation of `initialize` hook can cause infinite recursion too
easily. We stop invoking the method for safety, at the cost of less
flexibility.

The `initialize` methods (e.g. ones defined in `mrblib/10error.rb`) are
called only from `NoMethodError.new(args..)` forms.

`acc` meant `accumulator` but it is not an accumulator but just a
register position.

- `OP_ARYPUSH` now takes operand for the number of pushing elements
- the code generator consume the stack no more than `64` for `mruby/c`

Add n elements at once. Reduces instructions for huge array
initialization. In addition, `gen_value` function in `codegen.c` was
refactored and clarified.

Now `width` is limited to `INT16_MIN..INT16_MAX`.

For example, `{p:}` (means `{p:p}`) and `{String:}` (`{String:String}`)
should be allowed like CRuby.

`{x:, y:}` now is a syntax sugar of `{x: x, y: y}`.

This fix also includes the update of #4815 fix.

`acc` was used as an index of the receiver (if positive), or a flag for
methods implemented in C. We replace `regs[ci->acc]` by `ci[1].stack[0]`.
And renamed `acc` (originally meant accumulator position) to `cci`
(means callinfo for C implemented method).

The pool specified by `OP_STRING` (and `OP_SYMBOL`) should represent a
string, so that `IREP_TT_NFLAG` should be zero.

It generates a symbol by interning from the pool string.