Complete TodoMVC spec conforming implementation

[lukechu10]

This PR also includes some bug fixes and QOL improvements discovered while implementing TodoMVC:

• Implement std::fmt::Debug for Signal and StateHandle
• Parse keywords in component path
• Fix parsing of a component followed immediately by an interpolated value
  The following code previously failed to parse:

  template! {
      div {
          MyComponent()
          (interpolated_value)
      }
  }

  because the interpolated value would be treated as a function call of the previous component.
• Add an untrack utility for creating an untracked scope. This is useful when for example, calling a passed in closure and not wanting it to subscribe to current reactive scope.
• Create components inside an untracked scope (fixes tracking bugs when using template! inside interpolated values).
• Implement PartialEq for Signal and StateHandle.
• Implement Serialize and Deserialize for Signal and StateHandle only when the serde feature is enabled.