Fix panic with custom number literals

Fixed Issue #18. The bug was spotted by fuzzing.

Constants

• Added constants in terms of constant operators
• Default operators now contain π as PI and Euler's number as E.
• BREAKING - interface of Operator changed
• Fixed compilation of x-1+2.

Traits and serializable custom operators

• BREAKING the interface all over the place
• Expressions FlatEx and OwnedFlatEx now implement the trait Express and can be created in a unified way.
• Custom operators can be serialized with serde. To this end we introduced an operator-factory-trait and a macro for reduced boilerplate. Operator factories are used as generic parameters of the expression types.
• Parsing functions refactored

Hotfix missing trait implementations

Dervied standard trait implementations for OwnedFlatEx.

Owned expression

Added a new expression type that takes longer during parsing but does not need a lifetime parameter.

Basic serde support

• Added basic serde support via un-parsing/re-parsing.
• Fixed panic where both operators are None.
• Removed some duplicate checks after checking pre-conditions.

Bugfix outer derivative of -

• fixed computation of outer derivative for unary operators - and +, #16
• fixed DeepEx::is_num, #15
• fixed variable names with spaces, #14
• fixed documentation

Partial Derivatives

• Partial derivatives of expressions can be computed.
• BREAKING CHANGE! Variables' values now have to be passed to FlatEx::eval according to the alphatical order of the variable names instead of the order of occurence in the string to be parsed.
• Fixed panics triggered by garbage-in with the help of fuzzing

Hotfix 32 nodes

Fixed that flat expressions with more than 32 nodes panick, see Issue #6.

Display unparsed expression

• implemented unparse that transforms expressions, i.e., instances of FlatEx, into a string, see also discussion in #1
• implemented display in terms of unparse