Avoid creating temporary objects in read_irep_record_1()

[dearblue]

Since the irep pool element is now NUL terminated, break compatibility for mruby binary format.

[matz]

I like the idea, but I worry about the compatibility issue. Let me investigate the idea for a while.

[shuujii]

Null termination seems necessary for mrb_cstr_to_dbl to request it.

In most cases, if null termination is performed on the area allocated by MRB_ALLOCV (#4853), it can be realized without heap allocation or object creation, I think.

Alternatively, we may be able to use mrb_str_to_dbl with the following treatments:

1. Perform null termination using MRB_ALLOCV instead of RSTRING_CSTR in mrb_str_to_dbl.
2. Introduce something like CRuby's fake string (using RString on the stack).

FYI, there is the following bug (unreported). I think this can be fixed via 1.

$ ruby -e 'p "1\x002".to_f'
1.0

$ mruby -e 'p "1\x002".to_f'
trace (most recent call last):
-e:1: string contains null byte (ArgumentError)

[dearblue]

Here are some things I want to appeal to in this proposal:

1. Work object creation is reduced (= GC is reduced).
2. Reduced transfer between memories.
3. String literals are already NUL-terminated and can be extracted as C strings.

(1) and (2) can be especially effective on slow devices because they affect irep read time.
It's now that I came up with (3). 😸

[matz]

111045e is my attempt to address this issue that:

• Avoid creating temporary strings
• Do not change the binary format
• "1\x002".to_f works OK
• Float("0x10") now works

[dearblue]

I think it is a good change. Thank you!