Allow operators as symbols in various places

[Aloso]

This allows operators such as + or % in a few places where previously only symbols were allowed:

• In variable name in assignments: let + = "hello"
• As function name in a function call: + 3 4 (- 2 5)
• In map keys and values: let foo = { +=% }
• As indices: foo@+

In all other places, the operator must be wrapped in parentheses:

echo 3 (+) 5  # prints the integer 3, the addition operator and the integer 5
(+)@-         # accesses the key `-` of the object `+`

Operators can still be used in infix position:

> 3 + 5 == (+ 3 5)
True
> fmt @ dark @ blue == (@ fmt dark blue)
True

@, | and >> are turned into operators. They were classified as punctuation before, which doesn't make sense because unlike punctuation they can be overloaded.

! is added, which I apparently forgot to add to the tokenizer.

Builtin operators are renamed to the respective symbol. E.g. __add__ is now +, and redirect-to is now >>.

[adam-mcdaniel]

I much prefer defining operators by their actual symbol names instead of names like __add__, this is great!