ENH: Expand CIFTI2 constants to use a synonym recoder

[effigies]

For increased flexibility in the CIFTI2 axes API, this creates a set of synonyms for different CIFTI structures, mapping them onto the CIFTI_STRUCTURE_*.

Used for resolving synonyms in caret spec files in #1090, but mergeable on its own.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (590b226) 92.19% compared to head (20bffc3) 92.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1256   +/-   ##
=======================================
  Coverage   92.19%   92.19%           
=======================================
  Files          99       99           
  Lines       12456    12458    +2     
  Branches     2559     2560    +1     
=======================================
+ Hits        11484    11486    +2     
  Misses        648      648           
  Partials      324      324           

Files Changed	Coverage Δ
nibabel/cifti2/cifti2.py	96.87% <100.00%> (+0.01%)	⬆️
nibabel/cifti2/cifti2_axes.py	98.51% <100.00%> (ø)

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[effigies]

@mgxd Would you mind having a look when you get a chance?

[mgxd]

Few things, otherwise looks good