Prefer explicit construction to coercion in record initialization

While we support initializing records via coercion from an expression
list, e.g.,

    local x: X = [$x1=1, $x2=2];

this can sometimes obscure the code to readers, e.g., when assigning to
value declared and typed elsewhere. The language runtime has a similar
overhead since instead of just constructing a known type it needs to
check at runtime that the coercion from the expression list is valid;
this can be slower than just writing the readible code in the first
place, see #4559.

With this patch we use explicit construction, e.g.,

    local x = X($x1=1, $x2=2);

scripts/base/frameworks/sumstats/main.zeek

@@ -312,7 +312,7 @@ event zeek_init() &priority=100000
 
 function init_resultval(r: Reducer): ResultVal
 	{
-	local rv: ResultVal = [$begin=network_time(), $end=network_time()];
+	local rv = ResultVal($begin=network_time(), $end=network_time());
 	hook init_resultval_hook(r, rv);
 	return rv;
 	}