add recipes for solar panel + calculator

As described in
#26 (comment)

`calculator` device provides `arith` capability.

data/entities.yaml

@@ -396,7 +396,7 @@
 - name: plasma cutter
   display:
     attr: device
-    char: '+'
+    char: '*'
   description:
   - "A plasma cutter uses a jet of hot plasma to cut through metal like
     butter.  Used via the 'salvage' command to cut apart old robots."
@@ -564,3 +564,14 @@
     uses the 'log' command)."
   properties: [portable]
   capabilities: [log]
+
+- name: calculator
+  display:
+    attr: device
+    char: '+'
+  description:
+  - "A calculator allows a robot to do basic arithmetic calculations:
+    addition, subtraction, multiplication, division, and
+    exponentiation."
+  properties: [portable]
+  capabilities: [arith]

data/recipes.yaml

@@ -115,6 +115,20 @@
   required:
   - [1, furnace]
 
+- in:
+  - [1, glass]
+  - [8, copper wire]
+  out:
+  - [1, solar panel]
+  required:
+  - [1, 3D printer]
+
+- in:
+  - [1, counter]
+  - [1, solar panel]
+  out:
+  - [1, calculator]
+
 - in:
   - [32, gear]
   - [6, copper wire]

src/Swarm/Language/Capability.hs

@@ -266,6 +266,12 @@ constCaps =
     Salvage -> [CSalvage]
     Reprogram -> [CReprogram]
     Drill -> [CDrill]
+    Neg -> [CArith]
+    Add -> [CArith]
+    Sub -> [CArith]
+    Mul -> [CArith]
+    Div -> [CArith]
+    Exp -> [CArith]
     -- Some God-like sensing abilities.
     As -> [CGod]
     -- String operations, which for now are enabled by CLog
@@ -277,7 +283,6 @@ constCaps =
     Appear -> [CAppear] -- paint?
     Whereami -> [CSenseloc] -- GPS?
     Random -> [CRandom] -- randomness device (with bitcoins)?
-    Neg -> [CArith] -- ALU? pocket calculator?
     Whoami -> [CWhoami] -- mirror, needs a recipe
 
     -- comparator?
@@ -289,11 +294,6 @@ constCaps =
     Geq -> [CCompare]
     And -> []
     Or -> []
-    Add -> [CArith]
-    Sub -> [CArith]
-    Mul -> [CArith]
-    Div -> [CArith]
-    Exp -> [CArith]
     -- Some more constants which *ought* to have their own capability but
     -- currently don't.
     Say -> []