[vm] Refactor handling of kernel data in compiler frontend

a) Remove 2 fields in our object representation:

  * PatchClass::library_data_
  * Library::kernel_data_

=> This saves O(#libraries + #patch-classes) which can amount up to
  10+ MB for big apps, as we save not just the slots but the
  [ExternalTypedData] objects they used to reference.

Instead we'll compute the KernelLibraryData when we need it by making a
[TypedDataView] of sub parts of the component.

b) We make a kernel binary be represented by a single [ExternalTypedData].

Whenever we need a sub-part of a kernel binary (e.g. one for each
component for concatinated kernels, or one for a library inside a
component) we use proper [TypedDataView]s for that.

c) As we sometimes need to create a view of only a particular library
within a component we need to find start/end of a library based on
index.

=> We store the library index instead of the library offset on
Library/PatchClass.
=> We can easily derive the start/end of a library from it's index by
looking at the kernel component encoding.

d) We make the [Reader] object work purely based on a pointer - instead
of making it have if/else when reading bytes (either from pointer or
from a view).

e) We make the [KernelProgramInfo] store the kernel_component and
various TD views into it.

TEST=ci

Change-Id: Ibe160881ff48635e834c3d647a977a144b5d0565
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/313561
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Slava Egorov <vegorov@google.com>

runtime/vm/raw_object_fields.cc

@@ -30,7 +30,6 @@ namespace dart {
   F(Class, invocation_dispatcher_cache_)                                       \
   F(PatchClass, wrapped_class_)                                                \
   F(PatchClass, script_)                                                       \
-  F(PatchClass, library_kernel_data_)                                          \
   F(Function, name_)                                                           \
   F(Function, owner_)                                                          \
   F(Function, signature_)                                                      \
@@ -64,7 +63,6 @@ namespace dart {
   F(Library, imports_)                                                         \
   F(Library, exports_)                                                         \
   F(Library, dependencies_)                                                    \
-  F(Library, kernel_data_)                                                     \
   F(Library, resolved_names_)                                                  \
   F(Library, exported_names_)                                                  \
   F(Library, loaded_scripts_)                                                  \
@@ -72,6 +70,7 @@ namespace dart {
   F(Namespace, show_names_)                                                    \
   F(Namespace, hide_names_)                                                    \
   F(Namespace, owner_)                                                         \
+  F(KernelProgramInfo, kernel_component_)                                      \
   F(KernelProgramInfo, string_offsets_)                                        \
   F(KernelProgramInfo, string_data_)                                           \
   F(KernelProgramInfo, canonical_names_)                                       \
@@ -82,7 +81,6 @@ namespace dart {
   F(KernelProgramInfo, constants_table_)                                       \
   F(KernelProgramInfo, libraries_cache_)                                       \
   F(KernelProgramInfo, classes_cache_)                                         \
-  F(KernelProgramInfo, retained_kernel_blob_)                                  \
   F(WeakSerializationReference, target_)                                       \
   F(WeakSerializationReference, replacement_)                                  \
   F(WeakArray, length_)                                                        \