Add support for in-memory certificates

[sagebind]

Pull in new bindings for the new blob APIs added to curl in 2020. Bindings were recently added in alexcrichton/curl-rust#384. This of course only works when using a fairly new curl version.

Addresses #89.

[codecov]

Codecov Report

Merging #320 (6a399fc) into master (6f1c7b6) will decrease coverage by 0.26%.
The diff coverage is 8.33%.

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
- Coverage   74.94%   74.67%   -0.27%     
==========================================
  Files          57       57              
  Lines        2993     3009      +16     
==========================================
+ Hits         2243     2247       +4     
- Misses        750      762      +12     

Impacted Files	Coverage Δ
src/config/ssl.rs	35.00% <8.33%> (-8.75%)	⬇️
src/error.rs	45.33% <0.00%> (+0.66%)	⬆️
src/handler.rs	71.10% <0.00%> (+1.37%)	⬆️

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[sagebind]

Once the new bindings are published in a new curl release we can merge this.