Move `Repl` to `Value.replicate`

Summary

Replace the standalone Repl(value, count) node with value.replicate(count).

Motivation

Repl is a rarely used construct (it's mostly useful for manual sign extension).

It is currently a first-class entity that has its own AST node and a name in the prelude, mostly for historical reasons (Repl(v, n) is analogous to Verilog's {x{n}}).

Repl does not need to be a first-class entity; Repl(x, n) is almost exactly equivalent to Cat(x for x in range(n)). It especially does not need a name in the prelude.

Guide-level explanation

Use of Repl is deprecated. To replicate a value multiple times, use value.replicate().

Reference-level explanation

Direct use of Repl is deprecated. Its implementation is replaced with def Repl(value, count): return Value.cast(value).replicate(count).

A function Value.replicate(count) is added. It is implemented as Cat(value for _ in range(count)). The Repl AST node is removed.

Drawbacks

Churn.
The proposed implementation makes Value.replicate valid on left-hand side of assignment, with potentially surprising behavior. However, this can be handled by prohibiting multiple assignment to the same bit of a signal in general.

Rationale and alternatives

Rationale:

Fewer names in the prelude is always good.
Unlike with Cat (where Cat() makes sense), Repl does not make sense as a standalone node any more than Part does (and we do not currently export Part).
Despite existing by analogy with {x{n}}, it is currently turned into a concatenation before it reaches the Verilog backend anyway, and any future work will have to reconstruct replication from concatenation in any case.
Repl being a dedicated node complicates AST processing for no reason.

Alternatives:

Do not do this.

The Verilog backend currently bitblasts what could be a replication. We could detect these and convert them to replications proper.
We could detect code like Cat(x, x).eq(0b11) and warn or reject it.

The Amaranth RFC Book