examples: Add parameterized design example

This example showcases the usage of proto_to_dslx tool in DSLX design
parameterization.
It defines a protobuf message containing design parameters:

* input width,
* output width

A textproto file with a variant of the design is provided.
Textproto file can be converted to DSLX file which is then imported
by top-level design in order to use the parameters.

The design itself has 2 inputs and one output. The widths of those are
controlled by parameters.

Verilog file for the variant can be generated with:

```
bazel build //xls/examples/parameterized:design_verilog
```

Signed-off-by: Pawel Czarnecki <pczarnecki@antmicro.com>

xls/examples/parameterized/BUILD

@@ -0,0 +1,58 @@
+load("//xls/build_rules:xls_utilities.bzl", "proto_to_dslx")
+load(
+    "//xls/build_rules:xls_build_defs.bzl",
+    "xls_dslx_ir",
+    "xls_dslx_library",
+    "xls_ir_opt_ir",
+    "xls_ir_verilog",
+)
+
+proto_library(
+    name = "proto_schema_lib",
+    srcs = [":param.proto"],
+)
+
+proto_to_dslx(
+    name = "parameters",
+    proto_schema = ":proto_schema_lib",
+    proto_name = "xls.DesignVariants",
+    textproto = ":parameters.textproto",
+)
+
+xls_dslx_library(
+    name = "parameters_lib",
+    srcs = [":parameters"],
+)
+
+xls_dslx_library(
+    name = "design_lib",
+    srcs = [
+	  "design.x"
+	],
+    deps = [":parameters_lib"],
+)
+
+xls_dslx_ir(
+    name = "design_ir",
+    dslx_top = "top",
+    ir_file = "desing.ir",
+    library = "design_lib",
+)
+
+xls_ir_opt_ir(
+    name = "design_opt_ir",
+    src = "desing.ir",
+    top = "__design__top",
+)
+
+xls_ir_verilog(
+    name = "design_verilog",
+    src = ":design_opt_ir.opt.ir",
+    codegen_args = {
+        "module_name": "top",
+        "delay_model": "unit",
+        "pipeline_stages": "1",
+        "use_system_verilog": "false",
+    },
+    verilog_file = "design.v",
+)

xls/examples/parameterized/design.x

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+import xls.examples.parameterized.parameters
+
+type DesignParameters = parameters::DesignParameters;
+
+fn add<INPUT_WIDTH: u32, OUTPUT_WIDTH: u32>(x: uN[INPUT_WIDTH], y: uN[INPUT_WIDTH]) -> uN[OUTPUT_WIDTH] {
+    let lhs = x as uN[OUTPUT_WIDTH];
+    let rhs = y as uN[OUTPUT_WIDTH];
+    lhs + rhs
+}
+
+fn top(x: u64, y: u64) -> u64 {
+  let variant: DesignParameters = parameters::parameters.variant[0];
+  let variant_input_width: u32 = variant.input_width;
+  let variant_output_width: u32 = variant.output_width;
+  let arg_x = x as uN[variant_input_width];
+  let arg_y = y as uN[variant_input_width];
+
+  add<variant_input_width, variant_output_width>(arg_x, arg_y) as u64
+}

xls/examples/parameterized/param.proto

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+syntax = "proto2";
+
+package xls;
+
+message DesignParameters {
+  optional uint32 input_width = 1;
+  optional uint32 output_width = 2;
+}
+
+message DesignVariants {
+  repeated DesignParameters variant = 1;
+}

xls/examples/parameterized/parameters.textproto

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+variant {
+  input_width: 1
+  output_width: 2
+}
+