(core) move data engine code to core

Summary:
this moves sandbox/grist to core, and adds a requirements.txt
file for reconstructing the content of sandbox/thirdparty.

Test Plan:
existing tests pass.
Tested core functionality manually.  Tested docker build manually.

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2563

sandbox/grist/match_counter.py

@@ -0,0 +1,31 @@
+"""
+Simple class which, given a sample, can quickly count the size of overlap with an iterable.
+All elements of sample must be hashable.
+
+This is mainly in its own file in order to be able to test and time possible alternative
+implementations.
+"""
+class MatchCounter(object):
+  def __init__(self, sample):
+    self.sample = set(sample)
+
+  def count_unique(self, iterable):
+    """
+    Returns the count of unique elements of iterable that are present in sample. The sample may
+    only contain hashable elements, so non-hashable elements of iterable are never counted.
+    """
+    # The simplest implementation is 5 times faster:
+    #     len(self.sample.intersection(iterable))
+    # but fails if iterable can ever contain non-hashable values (e.g. list). This is the next
+    # best alternative. Attempting to skip non-hashable values with `isinstance(v, Hashable)` is
+    # another order of magnitude slower.
+    seen = set()
+    for v in iterable:
+      try:
+        if v in self.sample:
+          seen.add(v)
+      except TypeError:
+        # Non-hashable values can't possibly be in self.sample, so just don't count those.
+        pass
+
+    return len(seen)