[Rust] updated SubGroup.generateDecoder() to resolve unnecessary cast…

… warning in generated code (#986)

* [Rust] updated code generator to prevent rust warning for "ambiguous glob re-exports"

* [Rust] defined new trait 'ActingVersion'

* fixed code formatting issues

* [Rust] fixed benchmarks

* [Rust] added test for issue 984

* [Rust] updated SubGroup.generateDecoder() to resolve unnecessary cast warning

---------

Co-authored-by: Michael Ward <mward@drw.com>

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/SubGroup.java

@@ -157,7 +157,7 @@ void generateDecoder(
         indent(sb, level - 1, "#[derive(Debug, Default)]\n");
         indent(sb, level - 1, "pub struct %s<P> {\n", name);
         indent(sb, level, "parent: Option<P>,\n");
-        indent(sb, level, "block_length: usize,\n");
+        indent(sb, level, "block_length: %s,\n", rustTypeName(blockLengthPrimitiveType));
         indent(sb, level, "count: %s,\n", rustTypeName(numInGroupPrimitiveType));
         indent(sb, level, "index: usize,\n");
         indent(sb, level, "offset: usize,\n");
@@ -177,7 +177,7 @@ void generateDecoder(
         indent(sb, level + 1, "mut parent: P,\n");
         indent(sb, level, ") -> Self {\n");
         indent(sb, level + 1, "let initial_offset = parent.get_limit();\n");
-        indent(sb, level + 1, "let block_length = parent.get_buf().get_%s_at(initial_offset) as usize;\n",
+        indent(sb, level + 1, "let block_length = parent.get_buf().get_%s_at(initial_offset);\n",
             rustTypeName(blockLengthPrimitiveType));
         indent(sb, level + 1, "let count = parent.get_buf().get_%s_at(initial_offset + %d);\n",
             rustTypeName(numInGroupPrimitiveType), numInGroupToken.offset());