Add peek channel operation

[magancarz]

This pull request adds a peek channel operation which allows viewing the first to-be-received channel value without removing it from the channel queue. This operation is also available in conditional and blocking/non-blocking variants and can be tested by running a bytecode or IR interpreter.

Simple example in DSLX:

let (tok, packet) = peek(join(), req_r);
let handle_packet = packet.id > u32:4;
let (tok, packet) = recv_if(tok, req_r, handle_packet, zero!<Packet>());

More complex usages such as packet filler with blocking peek and content based arbiter with non-blocking peek can be found in blocking_peek.x and content_based_arbiter.x respectively in the xls/examples directory.

Example IR with peek can be found in IR conversion golden file or generated by building peek_ir target in the examples directory.

• Add support for Verilog generation
• See how recv and peek operations implementation can be joined to avoid duplicating code
• Add C++ tests for peek operation support