Remove System.read_line and SKIP_read_line

[jberdine]

Convert remaining uses of System.read_line to IO.BufferedReader, and remove System.read_line and associated runtime system functions SKIP_read_line, SKIP_read_to_end and their support code.

Originally posted by @jberdine in #409 (comment)

[jberdine]

Note that skjson calls read_line from a closure passed to Context.runWithGc. So since IO.BufferedReader is mutable, it cannot be used there, and some redesign will be needed.

The use in skdb appears to require a mutable global, so also will need some redesign.