Improvements to auto-completion

• consider the actual fields (either given or default) when looking for completions
• complete into facets using substring matches
• sort completions by document count (in reverse order)
• handle completions with spaces in the JS code
• benchmark to check the cost of the expanded functionality

Benchmark outcome after this MR:

description	query_str	iterations	timing	timing before MR
Completion with 4 Characters...	Bund	100	27.944485ms	63.19µs
Completion with 3 Characters...	Bun	100	28.068975ms	1.802µs
Completion with 4 Characters and only a very few results...	XYZA	100	24.584101ms	2.757µs
Completion a long string containing utf8 characters...	/?~Ѧ𝙱ƇᗞΣℱԍҤ١𝔍К𝓛𝓜ƝȎ𝚸𝑄Ṛ𝓢ṮṺƲᏔꓫ𝚈𝚭𝜶Ꮟçძ𝑒𝖿𝗀ḧ𝗂𝐣ҝɭḿ𝕟𝐨𝝔𝕢ṛ𝓼тú𝔳ẃ⤬𝝲𝗓1234567890!@#$%^&*()-_=+[{]};:',<.>/?~𝖠Β𝒞𝘋𝙴𝓕ĢȞỈ𝕵ꓗʟ𝙼ℕ০𝚸𝗤ՀꓢṰǓⅤ𝔚Ⲭ𝑌𝙕𝘢𝕤	100	7.713µs	6.757µs
Completion with lots of results...	Wass	100	30.998016ms	153.028µs
Completion with lots of results and strange query...	/?~Ѧ𝙱ƇᗞΣℱԍҤ١𝔍К𝓛𝓜ƝȎ𝚸𝑄Ṛ𝓢ṮṺƲᏔꓫ𝚈𝚭𝜶 Wass	100	30.979525ms	151.76µs