i#1114 JIT optimization: fragment tree API

Commit log for first patchset:
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i#1114 JIT optimization: fragment tree API

Defines structs for the JIT fragment tree, along
with the API for general fragment accounting. Also
adds a stub for a tree unit test.
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[Byron, 2015-12-17 13:26:27]

The fragment tree is complete in my local branch, including a randomized unit
test that should hit all the corner cases. This CL just covers the internal tree
API, since it may take some iterations to work out all the details, especially
if we want to reuse the tree for other things.

https://codereview.appspot.com/281080043/diff/1/core/jit_opt.c
File core/jit_opt.c (right):

https://codereview.appspot.com/281080043/diff/1/core/jit_opt.c#newcode89
core/jit_opt.c:89: special_heap_free(tree->special_heap, tree->nil);
Should we walk the tree and remove all the remaining nodes here?

https://codereview.appspot.com/281080043/diff/1/core/jit_opt.c#newcode101
core/jit_opt.c:101: #if (0)
Some of these functions may not be used for JIT fragments, but I'm including
them since we discussed more general use of the tree for fragment accounting.
I'm thinking let's try to work out a flexible API now to avoid heavy refactoring
later on. Unused functions are currently excluded from the compile to avoid
warnings.

[Byron, 2015-12-19 06:22:12]

Commit log for latest patchset:
---------------
i#1114 JIT optimization: fragment tree API

Defines structs for the JIT fragment tree, along
with the API for general fragment accounting. Also
adds a stub for a tree unit test.

Review-URL: https://codereview.appspot.com/281080043
---------------

[Byron, 2015-12-19 06:25:07]

I'm removing the lookup_for_insert() and insert_at() functions because there's
nothing to gain--maintenance of the subtree max requires the same path traversal
as a regular insert(). 

Adding clear() for general usage.

[bruening, 2015-12-20 15:48:09]

LGTM

[Byron, 2015-12-20 18:18:47]

Committed as
https://github.com/DynamoRIO/dynamorio/commit/747d74a7569f44861c04d1051cdb869...

Final commit log: 
---------------
i#1114 JIT optimization: fragment tree API

Defines structs for the JIT fragment tree, along
with the API for general fragment accounting. Also
adds a stub for a tree unit test.

Review-URL: https://codereview.appspot.com/281080043
---------------