interp: fix resolution of methods on aliased types

The type.val field was always pointing to the final underlying type
for aliased types, defeating a possible match if a method was
attached to a type in between. Now the complete chain of aliases
is always preserved.

We have added an underlying() itype method which returns the underlying
type of a defined type (aliasT), even in the presence of multiple
indirections.

We have added a definedType function which checks if type t1 is
defined from type t2 or t2 defined from t1, required when checking
assignability of aliasT types.

Fixes traefik#1411.

PS: this is the 2nd attempt, as the first version traefik#1412 wasn't passing
_test/issue-1408.go as well. This PR does pass and supersedes traefik#1412.

interp: improve handling values and comparisons in interfaces

Fixes traefik#1347.

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.