Load kernel BTF spec once when creating a new collection

When creating a new collection without specifying the `KernelTypes` in
the `ProgramOptions`, the kernel BTF is implicitly loaded. For this the
`btf.LoadKernelSpec()` helper is used. Even though the spec is only
loaded once, this operation cause a lot of memory churn, as the kernel
spec is being copied each time the function get called. This becomes an
issue when a collection with lots of programs is loaded in a resource
limited environment.

This commit initializes an missing `KernelTypes` field with the BTF
kernel spec, getting rid of the subsequent calls of `BTF.Copy()`.

This greatly reduces the number of allocations/op when running the
`BenchmarkNewCollectionManyProgs` benchmark. Here are the results:

Previous:
BenchmarkNewCollectionManyProgs-4  12 98443414 ns/op 116779863 B/op 403964 allocs/op

With optimization:
BenchmarkNewCollectionManyProgs-4 184  5807742 ns/op   4134444 B/op  17325 allocs/op

collection.go

@@ -252,7 +252,11 @@ func (cs *CollectionSpec) Assign(to interface{}) error {
 // Returns an error if any of the fields can't be found, or
 // if the same Map or Program is assigned multiple times.
 func (cs *CollectionSpec) LoadAndAssign(to interface{}, opts *CollectionOptions) error {
-	loader, err := newCollectionLoader(cs, opts)
+	if opts == nil {
+		opts = &CollectionOptions{}
+	}
+
+	loader, err := newCollectionLoader(cs, *opts)
 	if err != nil {
 		return err
 	}
@@ -351,7 +355,7 @@ func NewCollection(spec *CollectionSpec) (*Collection, error) {
 // Omitting Collection.Close() during application shutdown is an error.
 // See the package documentation for details around Map and Program lifecycle.
 func NewCollectionWithOptions(spec *CollectionSpec, opts CollectionOptions) (*Collection, error) {
-	loader, err := newCollectionLoader(spec, &opts)
+	loader, err := newCollectionLoader(spec, opts)
 	if err != nil {
 		return nil, err
 	}
@@ -405,9 +409,13 @@ type collectionLoader struct {
 	vars     map[string]*Variable
 }
 
-func newCollectionLoader(coll *CollectionSpec, opts *CollectionOptions) (*collectionLoader, error) {
-	if opts == nil {
-		opts = &CollectionOptions{}
+func newCollectionLoader(coll *CollectionSpec, opts CollectionOptions) (*collectionLoader, error) {
+	if opts.Programs.KernelTypes == nil {
+		kernelSpec, err := btf.LoadKernelSpec()
+		if err != nil {
+			return nil, fmt.Errorf("cannot load kernel spec: %w", err)
+		}
+		opts.Programs.KernelTypes = kernelSpec
 	}
 
 	// Check for existing MapSpecs in the CollectionSpec for all provided replacement maps.
@@ -423,7 +431,7 @@ func newCollectionLoader(coll *CollectionSpec, opts *CollectionOptions) (*collec
 
 	return &collectionLoader{
 		coll,
-		opts,
+		&opts,
 		make(map[string]*Map),
 		make(map[string]*Program),
 		make(map[string]*Variable),