switch variablechange string to fasteval instruction, add context structure for compiling

doc/uml/lib/class.puml

@@ -12,6 +12,10 @@ package nom {
     interface Parser<I>
 }
 
+package fasteval {
+    enum Instruction
+}
+
 package parser {
     interface TokenParser<I>
     interface Token {
@@ -29,7 +33,7 @@ package parser {
     }
     struct VariableChange {
         +name: char,
-        +change: String
+        +change: Instruction
         -parser<'a, I, C: Into<char> + Clone>(variables: &'a [C]): impl TokenParser<I>
     }
     struct Loop {
@@ -90,9 +94,12 @@ package parser {
 
 package compiler {
     struct Context {
-        result: Vec<f64>
-        variables: HashMap<char, f64>
-        ?
+        +result: Vec<f64>
+        +variables: HashMap<char, f64>
+        +instrument: Instruction
+        +slopes: HashMap<char, Instruction>
+        +current_length(&self): f64
+        +render(&self, n: Option<u8>): Vec<f64>
     }
 }
 
@@ -109,5 +116,7 @@ parser.TokenParser --|> nomparser_locatedspan: Output = Self, Error = nom::Err<n
 nomparser_locatedspan ..|> parser.TokenParser: for any T
 parser.Slope --> parser.VariableChange
 parser.Token --> compiler.Context
+VariableChange --> Instruction
+Context --> Instruction
 
 @enduml