Remove implicit error type conversion from Stream::fold

[stepancheg]

In my experience it does more harm than good: you need to specify
error type explicitly too often. See e. g. patched test case: it
fails before this commit.

stream.fold(0, |a, b| ok(a + b))

Without explicit error type parameter of ok, compilation fails with:

error[E0283]: type annotations required: cannot resolve `u32: std::convert::From<_>`
  --> tests/stream.rs:59:27
   |
59 |     assert_done(|| list().fold(0, |a, b| ok(a + b)), Ok(6));
   |                           ^^^^

[alexcrichton]

cc #291, an issue this is related to

This'll need to hold off merging until 0.2 regardless, but I'd personally be in favor as I've been bitten by these generics too many times.

[alexcrichton]

Ok! Now that 0.2 is underway I was looking to merge this, but it looks like this has already happened in 0.2, so closing.