Refactored the abi package parsing and type handling. Relying mostly on
package reflect as opposed to most of our own type reflection. Our own
type reflection is still used however for cases such as Bytes and
FixedBytes (abi: bytes•).
This also inclused several fixes for slice handling of arbitrary and
fixed size for all supported types.
This also further removes implicit type casting such as assigning,
for example `[2]T{} = []T{1}` will fail, however `[2]T{} == []T{1, 2}`
(notice assigning *slice* to fixed size *array*). Assigning arrays to
slices will always succeed if they are of the same element type.
Incidentally also fixes#2379
Fixed up `[]byte` slice support such that `function print(bytes input)`
accepts `[]byte` as input and treats it as 1 element rather than a slice
of multiple elements.
Added support for variable length input parameters like `bytes` and
`strings`.
Removed old unmarshalling of return types: `abi.Call(...).([]byte)`.
This is now replaced by a new syntax:
```
var a []byte
err := abi.Call(&a, ...)
```
It also addresses a few issues with Bytes and Strings and can also
handle both fixed and arbitrary sized byte slices, including strings.
