Add tree sequence metadata and metadata_schema to C API

[benjeffery]

C API part of #534. Adds metadata and metadata_schema fields to table collection, with accessors on tree sequence. These store arbitrary bytes and are optional in the file format.

[codecov]

Codecov Report

Merging #642 into master will decrease coverage by 0.08%.
The diff coverage is 65.67%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
- Coverage   87.71%   87.62%   -0.09%     
==========================================
  Files          23       23              
  Lines       17360    17426      +66     
  Branches     3421     3432      +11     
==========================================
+ Hits        15227    15270      +43     
- Misses       1044     1058      +14     
- Partials     1089     1098       +9     

Flag	Coverage Δ
#c_tests	88.86% <65.67%> (-0.13%)	⬇️
#python_c_tests	91.08% <ø> (ø)
#python_tests	98.90% <ø> (ø)

Impacted Files	Coverage Δ
c/tskit/tables.c	79.07% <61.01%> (-0.26%)	⬇️
c/tskit/trees.c	90.66% <100.00%> (+0.02%)	⬆️

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[benjeffery]

Looks like I could get a little more coverage here, setting back to draft.

[benjeffery]

@jeromekelleher Added a test on the TS methods. Looks like the rest of the coverage misses are hard error paths. Ready for review.

[jeromekelleher]

LGTM, thanks @benjeffery. I'll update to fix a couple of small issues and tag for merging.

[benjeffery]

@jeromekelleher did you see my comment above about the casting to char?

[benjeffery]

Oops just seen the reply, ignore!

[jeromekelleher]

I seem to have messed the PR up here - I thought you were off today @benjeffery so I force pushed a couple of changes. Looks like I broke stuff.

Crap, I think I pulled in kastore updates by git commit -a.

[jeromekelleher]

I made the mess, I'll fix it up!

[benjeffery]

I'm off, but can quickly push the branch back to how it was?

[jeromekelleher]

I'm off, but can quickly push the branch back to how it was?

Na, thanks, it's an easy fix.