dwarf: initial DWARFv5 support (#363)

dwarf: initial DWARFv5 support (#363)

* dwarf: initial DWARFv5 support

* dwarf/structs: use Embed to select header layout

* dwarf/structs: DW_FORM_strx family

Not sure how best to handle 24-bit values yet.

* dwarf/structs: use IfThenElse

`If` alone wraps the else in a `Value`.

* dwarf/structs: DW_FORM_addrx family handling

* dwarf_expr: support DW_OP_addrx

Not complete, but gets readelf.py to the end of a single
binary.

* dwarf/constants: DW_UT_* constants

* dwarf/structs: fix some DW_FORMs

* elftools, test: plumbing for DWARFv5 sections

* dwarf/constants: fix typo

* dwarf/structs: re-add a comment that got squashed

* dwarf/structs: DWARFv5 table header scaffolding

* dwarf/constants: typo

* test: add a basic DWARFv5 test