Improve packed field decoding

[osa1]

• Inline _readPacked manually and _withLimit with a pragma to eliminate
  closure allocation and calls in packed decoding loops.

  • _readPacked is manually inlined as VM's inliner doesn't properly fold
    constants after the inlinings, see
    Closure and closure context field loads are not folded in AOT dart-lang/sdk#60068.

• Introduce PbList._addUnchecked to add to the list without checking the
  value for validity and list for mutability.

• When decoding a packed field, check the list mutability once, instead of for
  every element.

• When decoding a packed scalar field, don't check for value validity.

  For scalar fields we need to make sure the field value is not null, which is
  already guaranteed in the call sites as e.g. input.readDouble doesn't
  return nullable.

• Sprinkle a bunch of prefer-inlines to make sure VM will inline one liners.

Looking at the optimized flow graph, this should be quite fast compared to the
current version. However I can't benchmark this yet as none of the benchmarks
use packed fields.

I will update the benchmarks first, then revisit this PR.

---

TODO: I think "check" functions in PbLists can be completely eliminated for the scalars where we only check for nulls:

protobuf.dart/protobuf/lib/src/protobuf/field_error.dart

Lines 75 to 107 in d00f905

	CheckFunc getCheckFunction(int fieldType) {
	switch (fieldType & ~0x7) {
	case PbFieldType._BOOL_BIT:
	case PbFieldType._BYTES_BIT:
	case PbFieldType._STRING_BIT:
	case PbFieldType._DOUBLE_BIT:
	case PbFieldType._ENUM_BIT:
	case PbFieldType._GROUP_BIT:
	case PbFieldType._MESSAGE_BIT:
	case PbFieldType._INT64_BIT:
	case PbFieldType._SINT64_BIT:
	case PbFieldType._SFIXED64_BIT:
	case PbFieldType._UINT64_BIT:
	case PbFieldType._FIXED64_BIT:
	// We always use the full range of the same Dart type.
	// It's up to the caller to treat the Int64 as signed or unsigned.
	// See: https://github.com/google/protobuf.dart/issues/44
	return _checkNotNull;
	case PbFieldType._FLOAT_BIT:
	return _checkFloat;
	case PbFieldType._INT32_BIT:
	case PbFieldType._SINT32_BIT:
	case PbFieldType._SFIXED32_BIT:
	return _checkSigned32;
	case PbFieldType._UINT32_BIT:
	case PbFieldType._FIXED32_BIT:
	return _checkUnsigned32;
	}
	throw ArgumentError('check function not implemented: $fieldType');
	}

The generated API for these have non-nullable element type (e.g. PbList<int>) so null checks no longer do anything (since null safety).