Fixes #1491.

The proxy operator T() conversions and the proxy-to-proxy assignment operators passed the raw result of get() into allocating calls: as<T>() coercions on the conversion side, and set() implementations that allocate before capturing the value on the assignment side. When get() returns a freshly computed SEXP (an active binding or active RefClass field, a field access evaluated via $, an attribute expanded by Rf_getAttrib()), that object is referenced from nowhere the GC can see, and a collection inside the window frees it while still in use. See #1491 for the analysis, including why standard builds of recent R rarely surface this (internal protection in coerceVector() since R 4.1.0, and R_UnwindProtect() accidentally keeping eval results reachable via the continuation token).

Changes:

• api/meat/proxy.h: Shield the get() result in all thirteen operator T() conversions and in FieldProxy::operator=(const FieldProxy&).
• proxy/SlotProxy.h: same for const_SlotProxy::operator T() (defined inline) and SlotProxy::operator=(const SlotProxy&).
• proxy/Binding.h, proxy/NamesProxy.h: same for the proxy-to-proxy assignments.
• Binding::operator=(const Binding&) additionally had a latent compile error: its self-assignment guard *this != rhs is ambiguous (the template conversion operators make it match unrelated != overloads), so the operator failed to compile whenever it was actually instantiated. The guard now compares the environment and name directly; the new regression test is this operator's first instantiation.
• Reviewed but intentionally not changed: AttributeProxy::operator=(const AttributeProxy&) (its set() shields immediately upon entry) and generic_name_proxy::operator T() in vector/proxy.h (its get() returns a parent-reachable element).

Verification:

• On an ASan/UBSan build of R-devel (r90339) with structure checking, the repro from proxy conversion and assignment operators use unprotected fresh SEXPs #1491 fails with unprotected object (0x...) encountered (was ENVSXP) against current master and runs clean with this patch.
• New regression tests exercise FieldProxy and Binding copies from actively-computed values under gctorture(); they pass on standard builds and guard the fix on protect-checking builds such as CRAN's ASan setup.

Checklist

• Code compiles correctly
• R CMD check still passes all tests (checked locally with RunAllRcppTests=yes on R 4.6.1; --no-manual --no-vignettes, so the only flagged items were two vignette-packaging warnings from the local --no-build-vignettes build)
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog