print: bypass the multiline mode of pretty_string_literal for merely trailing \n.

[eddyb]

At some point I added a multiline pretty string format using Rust-compatible string continuations, i.e. \ at the end of a line ignores all whitespace following it, so these are equivalent Rust literals:

"foo\
         \n"

"foo\n"

"foo
"

(the last one is never used by spirt::pretty, because it cannot be indented without changing the meaning of the string literal, and spirt::print::pretty assumes everything can be indented)

This PR ignores trailing \n when determining whether the multiline format should be used, i.e. only \n in the middle of a string will use \ continuations (with the \n moved to the start of following lines).

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Quick example (the very one that made me think it'd be worth a minor aesthetic improvement):

Before	After