btf: introduce caching string table to speed up ext info loading

[paulcacheux]

https://github.com/cilium/ebpf/pull/1772 introduced lazy loading of strings from string tables. It helps a lot when querying types, but when parsing line infos, we end up querying multiple times the same offest (especially the file name), and we currently create a new string each time.

This PR deduplicates this allocation by caching the string when parsing line info.

Benchmarks:

$ go test -exec=sudo -benchmem -run=^\$ -bench=^BenchmarkELF github.com/cilium/ebpf

With the elf file present in this repo (testdata/loader-el.elf):

goos: linux
goarch: arm64
pkg: github.com/cilium/ebpf
            │ before.txt  │           after.txt           │
            │   sec/op    │   sec/op     vs base          │
ELFLoader-4   379.4µ ± 4%   377.9µ ± 2%  ~ (p=0.912 n=10)

            │  before.txt  │              after.txt              │
            │     B/op     │     B/op      vs base               │
ELFLoader-4   187.8Ki ± 0%   186.8Ki ± 0%  -0.49% (p=0.000 n=10)

            │ before.txt  │             after.txt              │
            │  allocs/op  │  allocs/op   vs base               │
ELFLoader-4   1.470k ± 0%   1.402k ± 0%  -4.63% (p=0.000 n=10)

On a 18MiB elf file:

goos: linux
goarch: arm64
pkg: github.com/cilium/ebpf
            │ before.txt  │             after.txt              │
            │   sec/op    │   sec/op     vs base               │
ELFLoader-4   83.81m ± 2%   79.17m ± 2%  -5.54% (p=0.000 n=10)

            │  before.txt  │              after.txt               │
            │     B/op     │     B/op      vs base                │
ELFLoader-4   88.81Mi ± 0%   67.67Mi ± 0%  -23.80% (p=0.000 n=10)

            │  before.txt  │              after.txt              │
            │  allocs/op   │  allocs/op   vs base                │
ELFLoader-4   1132.2k ± 0%   834.3k ± 0%  -26.31% (p=0.000 n=10)