eval: add benchmarks

[pgavlin]

These changes add four benchmarks for the evaluator: one for the evalutator itself, one focused on fn::open, one focused on environment loading, and one for the entire stack. Each benchmark uses the same set of environments with different parameters for the time taken by an fn::open call or an environment load.

Local results are below.

goos: darwin
goarch: arm64
pkg: github.com/pulumi/esc/eval
cpu: Apple M1 Max
BenchmarkEval-10           	    196	  6029274 ns/op	4626576 B/op	  23578 allocs/op
BenchmarkEval-10           	    195	  6051825 ns/op	4625788 B/op	  23576 allocs/op
BenchmarkEval-10           	    195	  6041508 ns/op	4625803 B/op	  23576 allocs/op
BenchmarkEval-10           	    195	  6070728 ns/op	4625819 B/op	  23577 allocs/op
BenchmarkEval-10           	    194	  6065964 ns/op	4625696 B/op	  23577 allocs/op
BenchmarkEval-10           	    196	  6075660 ns/op	4626653 B/op	  23578 allocs/op
BenchmarkEval-10           	    195	  6161176 ns/op	4626057 B/op	  23578 allocs/op
BenchmarkEval-10           	    194	  6123274 ns/op	4625977 B/op	  23577 allocs/op
BenchmarkEval-10           	    194	  6098131 ns/op	4626670 B/op	  23578 allocs/op
BenchmarkEval-10           	    196	  6064363 ns/op	4626654 B/op	  23579 allocs/op
BenchmarkEvalOpen-10       	      9	124478532 ns/op	4629023 B/op	  23614 allocs/op
BenchmarkEvalOpen-10       	      9	124931037 ns/op	4630025 B/op	  23609 allocs/op
BenchmarkEvalOpen-10       	      9	123642606 ns/op	4634537 B/op	  23618 allocs/op
BenchmarkEvalOpen-10       	      9	124292352 ns/op	4631007 B/op	  23611 allocs/op
BenchmarkEvalOpen-10       	      9	122027546 ns/op	4633108 B/op	  23617 allocs/op
BenchmarkEvalOpen-10       	      9	123727792 ns/op	4630472 B/op	  23613 allocs/op
BenchmarkEvalOpen-10       	      9	121888051 ns/op	4630566 B/op	  23617 allocs/op
BenchmarkEvalOpen-10       	      9	123239685 ns/op	4630572 B/op	  23619 allocs/op
BenchmarkEvalOpen-10       	      9	124360343 ns/op	4631780 B/op	  23616 allocs/op
BenchmarkEvalOpen-10       	      9	122856963 ns/op	4631718 B/op	  23613 allocs/op
BenchmarkEvalEnvLoad-10    	      4	305906031 ns/op	4634742 B/op	  23643 allocs/op
BenchmarkEvalEnvLoad-10    	      4	304883292 ns/op	4636056 B/op	  23644 allocs/op
BenchmarkEvalEnvLoad-10    	      4	309884938 ns/op	4637584 B/op	  23643 allocs/op
BenchmarkEvalEnvLoad-10    	      4	301879375 ns/op	4634852 B/op	  23632 allocs/op
BenchmarkEvalEnvLoad-10    	      4	301303094 ns/op	4632002 B/op	  23639 allocs/op
BenchmarkEvalEnvLoad-10    	      4	303479062 ns/op	4638610 B/op	  23647 allocs/op
BenchmarkEvalEnvLoad-10    	      4	303368458 ns/op	4634654 B/op	  23635 allocs/op
BenchmarkEvalEnvLoad-10    	      4	304764969 ns/op	4637982 B/op	  23655 allocs/op
BenchmarkEvalEnvLoad-10    	      4	308650562 ns/op	4636046 B/op	  23640 allocs/op
BenchmarkEvalEnvLoad-10    	      4	305385281 ns/op	4629074 B/op	  23628 allocs/op
BenchmarkEvalAll-10        	      3	421889528 ns/op	4647314 B/op	  23686 allocs/op
BenchmarkEvalAll-10        	      3	412783556 ns/op	4639096 B/op	  23683 allocs/op
BenchmarkEvalAll-10        	      3	418679208 ns/op	4642520 B/op	  23682 allocs/op
BenchmarkEvalAll-10        	      3	413973958 ns/op	4632093 B/op	  23650 allocs/op
BenchmarkEvalAll-10        	      3	415022083 ns/op	4645818 B/op	  23679 allocs/op
BenchmarkEvalAll-10        	      3	421364694 ns/op	4633874 B/op	  23662 allocs/op
BenchmarkEvalAll-10        	      3	413825070 ns/op	4634840 B/op	  23661 allocs/op
BenchmarkEvalAll-10        	      3	424170639 ns/op	4630058 B/op	  23666 allocs/op
BenchmarkEvalAll-10        	      3	421530069 ns/op	4638128 B/op	  23684 allocs/op
BenchmarkEvalAll-10        	      3	416247500 ns/op	4638744 B/op	  23670 allocs/op