Use field instead of field size in Forms_SQRT2

[fingolfin]

... to prepare for a future with other field implementations

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.67%. Comparing base (d2c28a9) to head (b25971d).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/forms.gi	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   80.71%   80.67%   -0.04%     
==========================================
  Files           6        6              
  Lines        3660     3663       +3     
==========================================
+ Hits         2954     2955       +1     
- Misses        706      708       +2     

Files with missing lines	Coverage Δ
lib/forms.gi	82.24% <83.33%> (-0.07%)	⬇️

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