extmod/modre: Add support for start- and endpos.

Pattern objects have two additional parameters for the ::search and ::match
methods to define the starting and ending position of the subject within
the string to be searched.

This allows for searching a sub-string without creating a slice.  However,
one caveat of using the start-pos rather than a slice is that the start
anchor (`^`) remains anchored to the beginning of the text.

Signed-off-by: Jared Hancock <jared@greezybacon.me>

tests/extmod/re_start_end_pos.py

@@ -0,0 +1,78 @@
+# test start and end pos specification
+
+try:
+    import re
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+def print_groups(match):
+    print("----")
+    try:
+        if match is not None:
+            i = 0
+            while True:
+                print(match.group(i))
+                i += 1
+    except IndexError:
+        pass
+
+
+p = re.compile(r"o")
+m = p.match("dog")
+print_groups(m)
+
+m = p.match("dog", 1)
+print_groups(m)
+
+m = p.match("dog", 2)
+print_groups(m)
+
+# No match past end of input
+m = p.match("dog", 5)
+print_groups(m)
+
+m = p.match("dog", 0, 1)
+print_groups(m)
+
+# Caret only matches the actual beginning
+p = re.compile(r"^o")
+m = p.match("dog", 1)
+print_groups(m)
+
+# End at beginning means searching empty string
+p = re.compile(r"o")
+m = p.match("dog", 1, 1)
+print_groups(m)
+
+# End before the beginning doesn't match anything
+m = p.match("dog", 2, 1)
+print_groups(m)
+
+# Negative starting values don't crash
+m = p.search("dog", -2)
+print_groups(m)
+
+m = p.search("dog", -2, -5)
+print_groups(m)
+
+# Search also works
+print("--search")
+
+p = re.compile(r"o")
+m = p.search("dog")
+print_groups(m)
+
+m = p.search("dog", 1)
+print_groups(m)
+
+m = p.search("dog", 2)
+print_groups(m)
+
+# Negative starting values don't crash
+m = p.search("dog", -2)
+print_groups(m)
+
+m = p.search("dog", -2, -5)
+print_groups(m)