The only case that doesn't work is decoding a wrapper type from JSON
at the top level.  This doesn't make sense and probably no users do it
I changed it to throw.

* Avoid initializing primitive fields in layout_init

* Avoid initializing string/bytes/message fields in layout_init

* Lazily create map when needed

* Lazily create repeated fields

* Change layout_init to only do memcpy

* Fix test for php-7.0

* Fix conformance test where default value of string/message map is not encoded

* Fix test for zts

* Clean up

* Fix comments

Update GSON and errorprone

Update to 3.10.0 in docs

This way, we can avoid an unnecessary copy.

merge

Down integrate Google3 changes to Github

Installing BOM before running Java test

Add com.google.truth as test dependencies

This goes hand-in-hand with other dependencies like junit, but seems to
only have been used from util/pom.xml.

Add length checks to ExtensionCollection

A few things that don't work or aren't tested yet:
- wrappers at the top level
- equality checking for not-yet-expanded wrappers.

c# feature(RepeatedField): Capacity property to resize the internal array

Remove unused argument to avoid UB