If _subject is a string, read the string and evaluate the resulting expression.
This is similar to execute_string.
If _subject is a file, read each expression in the file and evaluate it,
returning the result of the evaluation of the last expression.
This is similar to execute_file
If _subject is anything else, send the object, usually an expression to
the evaluator. This is similar to execute_expr.
env_or_context is the environment to evaluate the expression in.
An env is just an object that contains the binding of local variables.
". result=5 />
"./> result=9 />
Execute can also take Water expressions to execute as the content argument:
2. result=5 />
Execute takes an executing_kind argument. This defaults to "ek_code"
"ek_code" Treat the _subject or content as an expression and execute it.
"ek_expression" Just returns the subject or the expression(s) in the content
"ek_data" Converts the _subject or content into "data".
"ek_string" If there's a content, just return that string.
else if there's no content, error.