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: rename string_list internal function to string_list instead of string_map
embedding: cef_string_utf8_cmp()
M1456, Implement MIME sniffing initial Step
Download Cargo over HTTPS. Fix #3582.
embedding: replace transmutes with manual casts in cef_string_utf8_cmp()
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
Don't overwrite redirected URL in script_task (fixes #3970)
Remove unused arguments from LoadComplete and LoadCompleteMsg
auto merge of #3971 : SimonSapin/servo/cargo-https, r=Ms2ger

Download from https://static-rust-lang-org.s3.amazonaws.com, which apparently does not require SNI? This bypasses the CloudFront CDN and goes to directly to S3.

CC @brson