gap-system / gap / 12371

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Extend BindConstant, MakeConstantGlobal to all object types

Currently we only support these on integer and boolean values. This patch
extends this to all object types. This would then simply be a "non-reversible"
version of `MakeReadOnlyGlobal`; i.e., unlike that, a "constant global
variables" can never be turned into a non-constant one. That is, it will
always refer to the same object.

Semantically this interpretation of `constness` is in line with other
languages, such as Julia, where `const` refers to the (re)assignability of a
variable identifier, *not* to the mutability of the object it refers to. (Note
that this is admittedly somewhat confusing in GAP, with
`MakeReadOnlyGlobal`/`MakeReadOnlyGVar` doing one thing and `MakeReadOnlyObj`
doing something orthogonal (and for now only in HPC-GAP....

Note that no further use is made of this for now; in particular, unlike for
small integer values, `true` and `false`, no attempt is made to "inline"
constant object in function definitions. However, interfaces to other
languages (such as the GAP-Julia interface) could exploit constness for
various optimizations.

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