Stick to single-line strings in rustflags construction

[dtolnay]

I found the use of multiline string literals with escaped newlines in this code to be unhelpful for readability.

Some of these are doing in 5 lines what could be done in 1 line.

honggfuzz-rs/src/bin/cargo-hfuzz.rs

Lines 293 to 297 in c5805fa

	rustflags.push_str(
	"\
	-C llvm-args=-sanitizer-coverage-trace-compares \
	",
	);

honggfuzz-rs/src/bin/cargo-hfuzz.rs

Line 271 in 4ef9de5

rustflags.push_str("-C llvm-args=-sanitizer-coverage-trace-compares ");

[dtolnay]

• Rebased to resolve conflict with Format with rustfmt #94