btf: reduce unnecessary copies when parsing BTF

[lmb]

We currently read all BTF into memory before starting decoding. This is wasteful,
especially for the large-ish string table. Be a bit more clever to remove intermediate
copies.

name            old time/op    new time/op    delta
ParseVmlinux-4     107ms ± 7%      77ms ±13%  -27.98%  (p=0.029 n=4+4)

name            old alloc/op   new alloc/op   delta
ParseVmlinux-4     119MB ± 0%      93MB ± 0%  -22.20%  (p=0.029 n=4+4)

name            old allocs/op  new allocs/op  delta
ParseVmlinux-4      786k ± 0%      786k ± 0%   -0.01%  (p=0.029 n=4+4)