embedding: begin cef string_multimap api

cef_string_multimap_alloc()
embedding: cef_string_multimap_size()
embedding: cef_string_multimap_find_count()
embedding: cef_string_multimap_append()
embedding: cef_string_multimap_enumerate()
embedding: cef_string_multimap_key()
embedding: cef_string_multimap_value()
embedding: cef_string_multimap_clear()
embedding: cef_string_multimap_free()
embedding: replace usage of deprecated find*() with get*()
address review comments
auto merge of #4025 : zmike/servo/embedding-string_multimap, r=jdm

@jdm @larsbergstrom