Make ExecutionSubject

[dplassgit]

E.g., so we can write:

 assertThat(sourceCode).throwsRuntimeException("match");
 assertThat(sourceCode).throwsGenerateException("match");
 assertThat(sourceCode).compiledOutput().hasExitCode(-1).contains("foo");
 assertThat(sourceCode).withNoOptimization().compiledOutput().contains("foo");
 assertThat(sourceCode).interpretedOutput().isEqualTo("bar");
 assertThat(sourceCode).compiledOutputEqualsInterpreted();