btf: introduce Bits type and remove Int.Bits and Int.Offset

[lmb]

btf: introduce Bits type

BTF expresses some lengths in bits, others in bytes. So far we don't have a
good convention how to distinguish the two units. Add a Bits type to make
the distinction clearer.

btf: remove Int.Bits and Int.Offset

Unfortunately BTF has two ways to encode bitfields. You can
create a Member and set Offset / BitfieldSize or create an Int that
has the offset and bitfield size baked in. The background for this
can be found in [1].

Remove Int.Bits and Int.Offset in favour of Member.BitfieldSize and
Member.Offset. Also get rid of a bunch of incorrect checks for bitfields,
which we didn't catch due to the weird encoding. This cleans up the API,
but may make our lives worse if we end up having to support writing out
legacy bitfields. Let's hope it doesn't come to that.

The performance of parsing vmlinux hasn't changed dramatically:

    name            old time/op    new time/op    delta
    ParseVmlinux-4    62.1ms ± 1%    61.8ms ± 2%    ~     (p=0.686 n=4+4)

    name            old alloc/op   new alloc/op   delta
    ParseVmlinux-4    44.2MB ± 0%    44.7MB ± 0%  +1.14%  (p=0.029 n=4+4)

    name            old allocs/op  new allocs/op  delta
    ParseVmlinux-4      718k ± 0%      718k ± 0%  +0.00%  (p=0.029 n=4+4)

1: https://github.com/torvalds/linux/commit/9d5f9f701b1891466fb3dbb1806ad97716f95cc3

[lmb]

Added TestInflateLegacyBitfield after discussing with Timo.