refactor & fixed assembler path handling

Cargo.lock

@@ -236,6 +236,8 @@ name = "assembler"
 version = "0.2.0"
 dependencies = [
  "common",
+ "num_cpus",
+ "threadpool",
 ]
 
 [[package]]
@@ -1951,6 +1953,16 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "num_cpus"
+version = "1.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "91df4bbde75afed763b708b7eee1e8e7651e02d97f6d5dd763e89367e957b23b"
+dependencies = [
+ "hermit-abi",
+ "libc",
+]
+
 [[package]]
 name = "num_enum"
 version = "0.7.3"
@@ -3041,6 +3053,15 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "threadpool"
+version = "1.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d050e60b33d41c19108b32cea32164033a9013fe3b46cbd4457559bfbf77afaa"
+dependencies = [
+ "num_cpus",
+]
+
 [[package]]
 name = "tiff"
 version = "0.9.1"