interp: handle struct with multiple recursive fields (traefik#1372)

* interp: handle struct with multiple recursive fields

In case of a recursive struct with several recursive fields of
different type, only the first one was properly fixed when
constructing the corresponding reflect type. We now memorize and
process all fields at the same depth level.

Fixes traefik#1371.

* Update interp/type.go

Co-authored-by: mpl <[email protected]>

* fix lint

* fix comment

Co-authored-by: mpl <[email protected]>

interp: fix wrapping of returned closure passed to runtime

Fixes traefik#1333.

interp: fix setting of interface value from nested function calls

Fixes traefik#1320 and traefik#1294.

interp: fix populating array of interfaces

Fixes traefik#1308.

interp: do not export RealFS, used internally only

stdlib: remove wrapper of runtime/cgo

Temporarily removing this wrapper which causes a failure
on freebsd system at build.

Fixes traefik#1221.

feat: update stdlib mapping for go1.17

* Drop go1.15
* Generate go1.17
* Update minimum extract version
* Update the CI config

interp: fix interface wrapper generation

Add early detection of cases where no wrapper is necessary because
the value type already implements the target interface.

It should both increase performances by avoiding the wrapper overhead,
and fix errors due to replacing valid values by incomplete wrappers,
caused by the presence of private methods in the interface definition,
as in traefik#1191.

Fixes traefik#1191.

interp: fix append of values to an array of interface objects

Interface wrappers were not properly generated.

Fixes traefik#1185

interp: improve handling of interface values

In selector resolution, struct field matching now precedes
method matching. Before struct field matching could be skipped
in case of a matching method, which is incorrect, as demontrated
by _test/issue-1156.go.

Field lookup has been fixed to operate on recursive structures.

Concrete type values are derived when filling a receiver for
interface methods.

LookupBinField has been fixed to skip non struct values.

LookupMethod has been fixed to iterate on interface values as
well as concrete type values.

Fixes traefik#1156.