bug#69749: (bindat-type sint 32 t) -> unbound variable bitlen

[Stefan Monnier]

>> -- file bd.el
>> (require 'bindat)
>> (bindat-pack (bindat-type sint 32 t) 42)
>> -- 

When you open this file in Emacs≥28, the modeline will have an orange
"/d" which is meant to warn you that something's amiss.
Click on it, and your problem will be solved :-)

>> Symbol’s value as variable is void: bitlen
>>
>> This bug is difficult to understand. Run from an IELM buffer, this code 
>> works as expected. The macroexpansion of the bindat-type declaration 
>> includes a binding for bitlen.

Your code ends up (via the `sint` of `bindat-type`) relying on the
`lexical-binding` dialect of ELisp, which is enabled by default in
`ielm` but not in your file.


        Stefan