SI-1945 REPL accretes lines to fix syntax

On a syntax error, if the last line compiled without
error, REPL will concatenate the two lines and try to
compile the result.

The use case is splitting an if/else over two lines
of input.

REPL will not retry on typer errors: that feature
would allow the use case currently handled by looking
for leading dot. For example, "abc"\nlength would work
as postfix invocation.

It does assist dot-completion in the case of extra
whitespace: "abc"\n .length works by accretion.