We previously used serde's stream deserializer to read json blobs from
the cargo output. It has an issue though: If the deserializer encounters
invalid input, it gets stuck reporting the same error again and again
because it is unable to foward over the input until it reaches a new
valid object.
Reading a line at a time and manually deserializing fixes this issue,
because cargo makes sure to only outpu one json blob per line, so should
we encounter invalid input, we can just skip a line and continue.
The main reason this would happen is stray printf-debugging in
procedural macros, so we still report that an error occured, but we
handle it gracefully now.
Fixes#2935
