Optimization for layout_init()

[haberman]

This avoids most of the work of layout_init() by creating a template object we can memcpy() from when initializing a new object.

There is a bit of complication since we must eagerly allocate maps and repeated fields. So we have to dup these still in layout_init().

In the benchmark on my machine this takes us from 8 seconds to 2.3 seconds, or a 70% reduction in runtime overall.

[fables-tales]

Initially this looks good for ads protos, just gathering some data on our large data set now!

[haberman]

I'm getting the following crash on Mac Ruby 2.3. I've reproduced this locally:

-- C level backtrace information -------------------------------------------
0   ruby                                0x000000010bd7cb4d rb_vm_bugreport + 381
1   ruby                                0x000000010bc049ef rb_bug_context + 447
2   ruby                                0x000000010bce3b74 sigsegv + 68
3   libsystem_platform.dylib            0x00007fff5ffc9b5d _sigtramp + 29
4   ruby                                0x000000010bc30d71 gc_mark_ptr + 81
5   protobuf_c.bundle                   0x000000010c0759bc layout_mark + 124
6   ruby                                0x000000010bc2f9d8 gc_marks_rest + 88
7   ruby                                0x000000010bc2e160 gc_start + 2880
8   ruby                                0x000000010bc316f8 garbage_collect_with_gvl + 216
9   ruby                                0x000000010bc2959d objspace_xmalloc + 141
10  ruby                                0x000000010bd33609 generic_ivar_update + 169
11  ruby                                0x000000010bcede3c st_update + 700
12  ruby                                0x000000010bd2e01c rb_ivar_set + 460
13  protobuf_c.bundle                   0x000000010c076580 Message_alloc + 144
14  ruby                                0x000000010bc6a195 rb_obj_alloc + 117
15  ruby                                0x000000010bc6b118 rb_class_new_instance + 24
16  ruby                                0x000000010bd72832 vm_call_cfunc + 290
17  ruby                                0x000000010bd5c15f vm_exec_core + 10655

The fact that this is in layout_mark() makes me worry that the crash might already be on master.

[haberman]

I did some testing and I have been unable to reproduce this on master, but it reproduces relatively easily on this branch. So it appears the bug is in this PR after all (which is good -- I can fix it before merging).

[haberman]

The crash that I was seeing earlier has been fixed in the latest commit. This is ready for review.