Add/Improve Debugging Features

Ideas/Tasks around improving the ability of developers to debug the programs they write using HEIR.

* Plaintext Execution Mode (i.e., run only arithmetization, then run resulting program in plaintext space Z_p, as discussed in the last office hours)
* "Decrypt-after-each" mode (kind of automating a common form of "printf() debugging")
* add secret_key as argument for eval
* after each HE operation (or maybe at granularity of each secret.generic after secret-distribute-generic?), decrypt the resulting ciphertext.
* compare the decrypted result with the expected plaintext result
* Report any deviations
* #1143