Tweak SqrtField_objectify; fix mutability issue

[fingolfin]

• Declare SqrtField_objectify using BindGlobal
• Drop type argument of SqrtField_objectify, it is always SqrtFieldType
• Remove one level of nesting for the elm argument to SqrtField_objectify: so instead of SqrtField_objectify([x]), now write SqrtField_objectify(x)
• Finally, don't pass an immutable list as second argument to Objectify. This is trivial to achieve thanks to the previous point.

Note that passing immutable objects as second argument to Objectify was always illegal in GAP, but due to a bug this restriction was not properly enforced. As a result, code accidentally passing in immutable objects silently worked. Future versions of GAP will issue a warning for this, and one distant day perhaps even an error.

In the meantime, this patch prepares corelg for this eventuality, and even if that day never comes, it does no harm either.

[codecov[bot]]

Codecov Report

Merging #8 into master will not change coverage. The diff coverage is 90%.

@@           Coverage Diff           @@
##           master       #8   +/-   ##
=======================================
  Coverage   92.33%   92.33%           
=======================================
  Files          16       16           
  Lines      179708   179708           
=======================================
  Hits       165925   165925           
  Misses      13783    13783

Impacted Files	Coverage Δ
gap/sqrt.gi	77.07% <90%> (ø)	:arrow_up: