Remove unused `ModuleProgram`

[LesterEvSe]

While researching implementation of the mod keyword related to issue #318, I found an unnecessary ModuleProgram struct.

Researching further, I found that it is used inside current simplicity-webide. The main flow currently lives in program_tab.rs L66 and follows by the simple Web IDE P2PK example:

mod witness {
    const ALICE_SIGNATURE: [u8; 64] = 0xf12fb2ed7d1d42bf20fb6c8aff9f9f6bcd53632a80e4f50541dd0f2e41a987e74444624e143ee72a85473667a6af783d8b1577be5028568233f5d3f7c6379d41;
}

mod param {
    const ALICE_PUBLIC_KEY: u256 = 0x9bef8d556d80e43ae7e0becb3a7e6838b95defe45896ed6075bb9035d06c9964;
}

fn main() {
    jet::bip_0340_verify((param::ALICE_PUBLIC_KEY, jet::sig_all_hash()), witness::ALICE_SIGNATURE)
}

The current pipeline works as follows:

1. Parse arguments: Extract param block from SimplicityHL source, ignoring all other code (handled by SimplicityHL itself).
2. Compile: Run the SimplicityHL compiler with the parsed params, ignoring all mod keywords.
3. Parse witness: Extract witness data and attach it to the compiled program (handled by SimplicityHL too).

As a result, I think we need a better approach here and ModuleProgram does not seem necessary for it.