Add `StringLiteral#to_utf16` #3122

Job	Run time
Build interpreter	10m 10s
Test Interpreter	4m 18s
Test primitives_spec with interpreter	35s
Test std_spec with interpreter (0)	4m 0s
Test std_spec with interpreter (1)	2m 3s
Test std_spec with interpreter (2)	3m 2s
Test std_spec with interpreter (3)	2m 23s
	26m 31s

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