Bump versions to v2.0.0-rc.0

[jonasbb]

bors r+

[codecov]

Codecov Report

Merging #484 (9c2923e) into master (8a51f84) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #484   +/-   ##
=======================================
  Coverage   70.79%   70.79%           
=======================================
  Files          52       52           
  Lines        3592     3592           
=======================================
  Hits         2543     2543           
  Misses       1049     1049           

Impacted Files	Coverage Δ
serde_with/src/lib.rs	100.00% <ø> (ø)
serde_with/tests/version_numbers.rs	79.31% <ø> (ø)
serde_with_macros/src/lib.rs	0.00% <ø> (ø)
serde_with_macros/tests/version_numbers.rs	100.00% <ø> (ø)

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[bors]

Build succeeded:

• Rustsec Audit
• Tests result