A few small improvements for HashMap that will increase efficiency of updated:

a) Avoid creating a temporary HashTrieMap with 0/1 child elements in HashMapCollision1.updated0 by moving the code from HashMap1.updated into a private utility method. Similar to #6220 for HashSet.

b) Prevent creation of one-element HashTrieMap in HashTrieMap.removed0. Similar to #6197 for HashSet

c) Prevent creation of one-element HashMapCollision1 in HashMapCollision1.removed0. Similar to #6198

d) Adding assertion to HashTrieMap and HashMapCollision1 to check tree consistency to prevent regression of a,b or c.

e) Remove unnecessary checks for merger eq null in merged0. (While updated0 can still be called with a null merger, merged0 is guaranteed to be called with a non-null merger)