fix: reject function equality comparisons with clear error

[He-Pin]

Motivation

Jsonnet semantics dictate that functions cannot be compared for equality. sjsonnet silently returned false for f == g (two functions), diverging from go-jsonnet, C++ jsonnet, and jrsonnet which all raise runtime errors. This silent divergence masked bugs where users accidentally compared functions instead of their return values.

Modification

• Evaluator.scala: Add Val.Func checks in equal() for object field comparison (lazy-evaluated fields), array element comparison, and top-level scalar comparison
• Raise Error.fail("cannot test equality of functions") when both operands are Val.Func instances
• Handle asymmetric cases (func == non-func) by returning false after checking the non-func operand

Result

function(x) x == function(x) x now raises sjsonnet.Error: cannot test equality of functions, matching go-jsonnet, C++ jsonnet, and jrsonnet behavior.

Implementation	Result	Error message
go-jsonnet v0.22.0	Error	RUNTIME ERROR: Cannot test equality of functions
C++ jsonnet	Error	RUNTIME ERROR: cannot test equality of functions
jrsonnet v0.5.0-pre99	Error	runtime error: cannot test equality of functions
sjsonnet (before)	Silent false	(no error, returns false)
sjsonnet (after)	Error	sjsonnet.Error: cannot test equality of functions

Test plan

• function(x) x == function(x) x raises error
• function(x) x != function(x) x raises error
• Function equality in arrays: [f] == [g] raises error
• Function equality in objects: {a: f} == {a: g} raises error
• Non-function comparisons unchanged: 1 == 2, "a" == "b", etc.
• All existing tests pass

[He-Pin]

Closing: PR #959 has identical Evaluator.scala changes plus comprehensive test coverage (3 test files). This PR is a strict subset — keeping #959 instead to avoid duplicate work.