Enum string API

[geyslan]

Context: #218

commit 9f3d56d (HEAD -> enum-string-api)

feat: introduce enum string api

This adds the Name() method to MapType, BPFAttachType and BPFProgType,
which relies on libbpf_bpf_map_type_str(), libbpf_bpf_attach_type_str()
and libbpf_bpf_prog_type_str() respectively.

commit eb4b613

chore(prog): add BPFAttachType string API

commit 424283a

chore: assign constants with C enum values

This avoids wrong values if changes happen.

commit cd3df8d

chore(prog): instantiate bpfProgTypeToString

This avoids the need to instantiate the map in every call to
BPFProgType.String().

commit bc215c4

chore(prog): deprecate BPFProgType.Value()

commit 5e4fbbb

fix(map): return unspecified for unknown type