add wasmtime adapter and stdio wrappers

the stdio wrappers will not work on windows, but thats a whole other can
of worms anyway

crates/wasi-c2/src/ctx.rs

@@ -1,6 +1,7 @@
 use crate::dir::{DirCaps, DirEntry, WasiDir};
 use crate::file::{FileCaps, FileEntry, WasiFile};
 use crate::table::Table;
+use crate::Error;
 use std::cell::{RefCell, RefMut};
 use std::path::PathBuf;
 use std::rc::Rc;
@@ -10,6 +11,10 @@ pub struct WasiCtx {
 }
 
 impl WasiCtx {
+    pub fn builder() -> WasiCtxBuilder {
+        WasiCtxBuilder(WasiCtx::new())
+    }
+
     pub fn new() -> Self {
         WasiCtx {
             table: Rc::new(RefCell::new(Table::new())),
@@ -52,3 +57,36 @@ impl WasiCtx {
         self.table.borrow_mut()
     }
 }
+
+pub struct WasiCtxBuilder(WasiCtx);
+
+impl WasiCtxBuilder {
+    pub fn build(self) -> Result<WasiCtx, Error> {
+        Ok(self.0)
+    }
+    pub fn arg(&mut self, _arg: &str) -> &mut Self {
+        // Intentionally left blank. We do not handle arguments yet.
+        self
+    }
+    pub fn inherit_stdio(&mut self) -> &mut Self {
+        self.0.insert_file(
+            0,
+            Box::new(crate::stdio::stdin()),
+            FileCaps::READ,
+            FileCaps::READ,
+        );
+        self.0.insert_file(
+            1,
+            Box::new(crate::stdio::stdout()),
+            FileCaps::WRITE,
+            FileCaps::WRITE,
+        );
+        self.0.insert_file(
+            2,
+            Box::new(crate::stdio::stderr()),
+            FileCaps::WRITE,
+            FileCaps::WRITE,
+        );
+        self
+    }
+}