perf(syntax): simplify @rustIdentifiers

There didn't need to be a huge chain of syntax groups to check. Some of
them could be contained in other groups, and this should also help in
removing the need to check for reduntant groups until a group which
contains those groups is matched.

Author: Iron-E

syntax/rust.vim

@@ -67,11 +67,11 @@ syn keyword   rustExternCrate crate contained nextgroup=rustType,rustExternCrate
 syn match   rustExternCrateString /".*"\_s*as/ contained display nextgroup=rustType skipwhite transparent skipempty contains=rustString,rustOperator
 syn keyword   rustObsoleteExternMod mod contained nextgroup=rustType skipempty skipwhite
 
-syn match  rustIdentifier "\<\l\+\(_\l\+\)*\((\)\@!\>" contained contains=rustBoolean,rustSelf display
-syn match  rustType       "\<\(\u\l*\)\+\((\)\@!\>" contains=rustTypePrime contained display
-syn match  rustConstant   "\<\u\+\(_\u\+\)*\((\)\@!\>" contained display
-syn match  rustFuncName   "\<\w\+\(::\)\?\(<.*>\)\?\s*\((\)\@=\>" contains=rustEnumVariant,rustModPathSep,rustGenericRegion display
-syn cluster rustIdentifiers contains=@rustLifetimes,rustModPath,rustBuiltinType,rustEnum,rustTrait,rustEnumVariant,rustMacro,rustFuncName,rustConstant,rustIdentifier,rustType
+syn match  rustIdentifier "\<\l\+\(_\+\l\+\)*\>" contained contains=rustBoolean,rustSelf display
+syn match  rustConstant   "\<\u\+\(_\+\u\+\)*\>" contained display
+syn match  rustFuncName   "\<\w\+\(::\)\?\(<.*>\)\?\s*\((\)\@=\>" contains=rustModPathSep,rustGenericRegion display
+syn match  rustType       "\<\(\u\l*\)\+\>" contains=rustEnum,rustEnumVariant,rustTrait display
+syn cluster rustIdentifiers contains=@rustLifetimes,rustModPath,rustMacro,rustBuiltinType,rustType,rustFuncName,rustConstant,rustIdentifier
 
 syn region rustMacroRepeat matchgroup=rustMacroRepeatDelimiters start="$(" end="),\=[*+]" contains=TOP
 syn match rustMacroVariable "\$\w\+" display