embedding: begin cef string_map api

cef_string_map_alloc()
embedding: cef_string_map_size()
embedding: cef_string_map_append()
embedding: cef_string_map_find()
embedding: cef_string_map_key()
embedding: cef_string_map_value()
embedding: cef_string_map_clear()
embedding: cef_string_map_free()
embedding: address various review nitpicks for string_map
embedding: add eutil::slice_to_str() convenience function

converts a *u8+length to &str and returns c_int
embedding: replace all occurrences of slicing to &str with slice_to_str()
embedding: address most recent critic review
auto merge of #3947 : zmike/servo/embedding-string_map, r=jdm

implements a string map which is 100% identical to CEF

r+ @larsbergstrom  @jdm