Update signal-hook-tokio for tokio v1.0

[bdash]

This drops support for pre-1.0 versions of tokio, and removes the per-version features and modules.

Fixes #83.

[vorner]

Thank you! I somehow haven't managed to get around to this yet, so thanks for picking it up.

There are few details that need ironing out, but mostly this is what needed doing. Can you have a look at them?

[codecov-io]

Codecov Report

Merging #87 (b3ad9eb) into master (1f271a0) will decrease coverage by 1.78%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   63.05%   61.26%   -1.79%     
==========================================
  Files          26       27       +1     
  Lines        3762     3565     -197     
  Branches      145      181      +36     
==========================================
- Hits         2372     2184     -188     
+ Misses       1389     1380       -9     
  Partials        1        1              

Impacted Files	Coverage Δ
signal-hook-tokio/src/lib.rs	92.59% <84.61%> (+2.59%)	⬆️
signal-hook-tokio/tests/tests.rs	100.00% <100.00%> (ø)
src/flag.rs	74.41% <0.00%> (-17.01%)	⬇️
signal-hook-registry/src/half_lock.rs	75.36% <0.00%> (-2.98%)	⬇️
src/lib.rs	47.39% <0.00%> (-2.82%)	⬇️
build.rs	85.93% <0.00%> (-1.95%)	⬇️
src/low_level/siginfo.rs	70.96% <0.00%> (-1.59%)	⬇️
src/low_level/mod.rs	50.00% <0.00%> (ø)
src/low_level/signal_details.rs	27.27% <0.00%> (ø)
examples/print.rs	0.00% <0.00%> (ø)

[vorner]

Thank you :-). Looks good.

Do you want to follow up with that cleanup right away, or should I spin up a release?