Implement DW_AT_ranges writing

[bjorn3]

This is needed for https://github.com/bjorn3/rustc_codegen_cranelift/issues/166 line info, because I can't rely on faerie putting functions in a certain order. And gdb doesn't accept line info when neither DW_AT_low_pc nor DW_AT_high_pc/DW_AT_ranges is set.

[bjorn3]

Working on this. I only need to implement converting from the read version and test it.

[bjorn3]

I have got a failing test :( https://github.com/bjorn3/gimli/tree/impl_range_write

[philipc]

I haven't looked in detail, but probably need to convert the raw ranges. I can have another look tomorrow. Also feel free to open a WIP PR.