Fix slice out of bounds panic

fixes tidwall#196

Update the match package

This commit updates the match package to v1.0.3, which includes
a fix to an issue where a pattern with lots of repetitive stars
will increasingly slow down a Match operation.

Fixes tidwall#195

Fix bounds out of range panic

This commit fixes an issues where gjson panics when the input json
or path ends in incomplete quoted string.

fixes tidwall#192

Better safe integer range for numbers

This commit optimizes the safe float to int conversion function
by using the full -9007199254740991 to 9007199254740991 range as
suggested at:

https://tc39.es/ecma262/#sec-number.min_safe_integer

closes tidwall#174

Adjusted test

Updated pretty package

Remove custom appengine and js builds

This commit allows for gjson and sjson to be supported in
WebAssembly `wasm` projects, but breaks compatibility for
anyone using gjson with older Google App Engine and GopherJS.

Added new modifiers

`@flatten` Flattens an array with child arrays.
  [1,[2],[3,4],[5,[6,7]]] -> [1,2,3,4,5,[6,7]]
The {"deep":true} arg can be provide for deep flattening.
  [1,[2],[3,4],[5,[6,7]]] -> [1,2,3,4,5,6,7]
The original json is returned when the json is not an array.

`@join` Joins multiple objects into a single object.
  [{"first":"Tom"},{"last":"Smith"}] -> {"first","Tom","last":"Smith"}
The arg can be "true" to specify that duplicate keys should be preserved.
  [{"first":"Tom","age":37},{"age":41}] -> {"first","Tom","age":37,"age":41}
Without preserved keys:
  [{"first":"Tom","age":37},{"age":41}] -> {"first","Tom","age":41}
The original json is returned when the json is not an object.

`@valid` Ensures that the json is valid before moving on. An
empty string is returned when the json is not valid, otherwise
it returns the original json.

Removed unmarshalling

Removed unneeded comment