[Feature request] Add conditional loop parameter application syntax

[gitfede56]

Support a syntax like @parameter(condition) to apply the @parameter only if condition (which is known at compile-time) is true.

This is really syntactic sugar, so it's not a priority, but since I think it should be very easy to implement (it's basically a macro), it would reduce code duplication and make things clearer with all the associated benefits.

Practical example of unrolling a loop:

# Current:
@parameter
if nchunks <= 4:
    @parameter
    for chunk in range(nchunks):
        var idx = ...
        dest.store(idx, src.load[width=32](idx))
else:
    for chunk in range(nchunks):
        var idx = ...
        dest.store(idx, src.load[width=32](idx))

# Ideal:
@parameter(nchunks <= 4)
for chunk in range(nchunks):
    var idx = ...
    dest.store(idx, src.load[width=32](idx))

At the moment it only makes sense for loop unrolling I think, but maybe in the future there can be other situations in which can come in handy.