Add basic GuestString support to wiggle (#13)

* Add basic GuestString support to wiggle

This commit adds basic `GuestString` support to `wiggle`. `GuestString`
is a wrapper around `GuestArray<'_, u8>` array type which itself can
be made into either an owned (cloned) Rust `String` or borrowed as
a reference `&str`. In both cases, `GuestString` ensures that the
underlying bytes are valid Unicode code units, throwing a `InvalidUtf8`
error if not.

This commit adds support *only* for passing in strings as arguments
in WASI. Marshalling of the return arg has not yet been implemented.
I'm not even sure it's possible without multi-value return args
feature of Wasm. It's not a major setback especially since the WASI
spec (and this includes even the `ephemeral` snapshot) doesn't
return strings anywhere. They are only ever passed in as arguments
to interface functions.

It should be noted that error returned in case of invalid UTF-8
requires a lot more love as it doesn't include anything besides
flagging an event that the string contained an invalid Unicode code unit.

* Borrow all of string's memory including nul-byte

Borrow all of string's underlying memory including the nul-byte.
This perhaps might not have a tremendous impact on anything, but
since the nul-byte is technically part of the WASI string, we should
include it in the borrow as well.

* Fill in wiggle-generate blanks for strings

* Print to screen passed string in proptest

* Strings are PointerLengthPairs!

* Fix generation of strings in compound types

* Update test with simple string strategy

* Generate better test strings

* Finalise proptest for strings

* Fix formatting

* Update crates/runtime/src/memory/string.rs

Removes unnecessary comment in code

* Apply Pat's suggestion to wrap Utf8Error as error

crates/runtime/src/lib.rs

@@ -6,5 +6,8 @@ mod region;
 
 pub use error::GuestError;
 pub use guest_type::{GuestErrorType, GuestType, GuestTypeClone, GuestTypeCopy};
-pub use memory::{GuestArray, GuestMemory, GuestPtr, GuestPtrMut, GuestRef, GuestRefMut};
+pub use memory::{
+    GuestArray, GuestMemory, GuestPtr, GuestPtrMut, GuestRef, GuestRefMut, GuestString,
+    GuestStringRef,
+};
 pub use region::Region;