userfunc: allow constructing a single function

[tonistiigi]

Refactors previous internal functionality of decoding a single function so that functions can be optionally decoded individually.

This is useful then functions need to depend on each other and static analysis is performed to understand what variables/functions they depend on.

No functional changes to existing DecodeUserFunctions.

Signed-off-by: Tonis Tiigi [email protected]

[hashicorp-cla]

All committers have signed the CLA.

[tonistiigi]

@radeksimko @jbardin