ProfilingEvaluator: update ExprBox.name to print builtin function and operator names

[JoshRosen]

This PR implements a small quality-of-life improvement in ProfilingEvaluator, updating ExprBox.name so that it prints the names of built in functions and operators, making it easier to interpret profiler output.

For example, before we'd print:

[info] Profiling...
[info] Top 20 by time:
[info] - 1286ms ApplyBuiltin2 <some-path>/functions.jsonnet.TEMPLATE:162:15
[info] - 470ms ApplyBuiltin2 <some-path>/functions.jsonnet.TEMPLATE:152:15
[info] - 443ms BinaryOp <some-path>/functions.jsonnet.TEMPLATE:158:35
[info] - 307ms Lookup <some-path>/functions.jsonnet.TEMPLATE:158:42
[....]

With this PR's small change, that becomes:

[info] Top 20 by time:
[info] - 1161ms ApplyBuiltin2 (join) <some-path>/functions.jsonnet.TEMPLATE:162:15
[info] - 459ms ApplyBuiltin2 (join) <some-path>/functions.jsonnet.TEMPLATE:152:15
[info] - 439ms BinaryOp (+) <some-path>/functions.jsonnet.TEMPLATE:158:35
[info] - 311ms Lookup <some-path>/functions.jsonnet.TEMPLATE:158:42
[...]

[JoshRosen]

Don't merge this until #265 lands because I'll need to update this for the ApplyBuiltIn3/4 added there.