Speed up BindingReflections.getAugmentations()

Speed up BindingReflections.getAugmentations()

One of the ways in which we can acquire augmentations is through
AugmentationHolder interface. While this is handled through
AugmentationFieldGetter.getGetter(), that is not really efficient
as it requires indirection through multiple implementations.

Since AugmentationFieldGetter is a package-internal implementation
detail, factor the check out into BindingReflections, where it can
execute directly. This not only improves locality, but also allows
us to remove one AugmentationFieldGetter implementation, resulting
in bimorphic invocation.

Change-Id: I15584dbe6151a70d13a766f27b927f598a5d21ba
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>