Fix Error.mapM returning reversed list

[emersion]

This is (hopefully) the last episode of the long list of patches trying to fix
the reversed lists issues. I originally started to try to fix in 1, but I
realized that the fix is incorrect.

My previous patch changed how read_elf{32,64}_section_header_table' returned
its output: instead of putting the newly parsed element at the head of the list,
it appends it at the end. However this is wrong: the function first parses a
header, and then parses the remaining headers recursively. This means the
parsed element must become the head of the returned list.

I then tried to pin down the real issue from where I noticed it, in
elf_memory_image_of_elf64_file. In this function the sections are reversed,
in read_elf{32,64}_section_header_table' the order is correct, the issue is
between those two functions.

I ran into some issues with List.mapi: the standard OCaml version calls the
iterator in the normal order meaning side effects (such as printing a debug
line) are correctly triggered. However the Lem version calls the iterator in the
reverse order, which is confusing. I'll definitely investigate this later.

The problem lies in obtain_elf64_interpreted_sections: the section header
list it eats is in the correct order, but the list of sections it returns is
reversed. After some more debugging I realized Error.mapM reverses the lists
it gets called with.

This patch fixes this issue and removes the previous workarounds.