Underscore is a letter (#261)

or at least, it's a valid identifier name.

src/renamer.fs

@@ -74,8 +74,8 @@ module private RenamerImpl =
         )
 
         let mutable best = -10000, ""
-        // For performance, consider at most 25 candidates.
-        for word: string in candidates |> Seq.take 25 do
+        // For performance, consider at most 26 candidates.
+        for word: string in candidates |> Seq.take 26 do
             let firstLetter = word.[0]
             let lastLetter = word.[word.Length - 1]
             let mutable score = 0
@@ -353,7 +353,7 @@ module private RenamerImpl =
     let computeListOfNames text =
         let charCounts = Seq.countBy id text |> dict
         let count c = match charCounts.TryGetValue(c) with true, res -> res | _ -> 0
-        let letters = ['a'..'z'] @ ['A'..'Z']
+        let letters = ['a'..'z'] @ ['A'..'Z'] @ ['_']
         [
             // First, use most frequent letters
             yield! letters |> List.sortBy count |> List.rev |> List.map string

tests/unit/many_variables.expected

@@ -5,11 +5,11 @@
  "{"
    "float M=L,f=K,N=J,O=I,P=H,Q=G;"
    "int R=1,S=2,T=3,U=4,V=5,W=6,X=7,Y=8;"
-   "float Z=F,a=E,b=D,c=C,d=B,e=A;"
-   "int g=1,h=2,i=3,j=4,k=5,l=6,m=7,n=8;"
-   "float o=0.;"
-   "int p=1,q=2,r=3,s=4,t=5,u=6,v=7,w=8;"
-   "float x=0.;"
-   "int y=1,z=2,at=3,aa=4,ab=5,ac=6,ad=7,aA=8;"
-   "return Y+n+w+aA;"
+   "float Z=F,_=E,a=D,b=C,c=B,d=A;"
+   "int e=1,g=2,h=3,i=4,j=5,k=6,l=7,m=8;"
+   "float n=0.;"
+   "int o=1,p=2,q=3,r=4,s=5,t=6,u=7,v=8;"
+   "float w=0.;"
+   "int x=1,y=2,z=3,at=4,aa=5,ab=6,ac=7,aA=8;"
+   "return Y+m+v+aA;"
  "}",