Refactor calling convention settings. (#304)

Add a calling-convention setting to the `Flags` used as part of the
`TargetIsa`. This allows Cretonne code that generates calls to use the
correct convention, such as when emitting libcalls during legalization
or when the wasm frontend is decoding functions. This setting can be
overridden per-function.

This also adds "fast", "cold", and "fastcall" conventions, with "fast"
as the new default. Note that "fast" and "cold" are not intended to be
ABI-compatible across Cretonne versions.

This will also ensure Windows users will get an `unimplemented!` rather
than silent calling-convention mismatches, which reflects the fact that
Windows calling conventions are not yet implemented.

This also renames SpiderWASM, which isn't camel-case, to Baldrdash,
which is, and which is also a more relevant name.

lib/codegen/meta/gen_settings.py

@@ -19,6 +19,32 @@ except ImportError:
     pass
 
 
+def gen_to_and_from_str(ty, values, fmt):
+    # type: (str, Tuple[str, ...], srcgen.Formatter) -> None
+    """
+    Emit Display and FromStr implementations for enum settings.
+    """
+    with fmt.indented('impl fmt::Display for {} {{'.format(ty), '}'):
+        with fmt.indented(
+                'fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {',
+                '}'):
+            with fmt.indented('f.write_str(match *self {', '})'):
+                for v in values:
+                    fmt.line('{}::{} => "{}",'
+                             .format(ty, camel_case(v), v))
+
+    with fmt.indented('impl str::FromStr for {} {{'.format(ty), '}'):
+        fmt.line('type Err = ();')
+        with fmt.indented(
+                'fn from_str(s: &str) -> result::Result<Self, Self::Err> {',
+                '}'):
+            with fmt.indented('match s {', '}'):
+                for v in values:
+                    fmt.line('"{}" => Ok({}::{}),'
+                             .format(v, ty, camel_case(v)))
+                fmt.line('_ => Err(()),')
+
+
 def gen_enum_types(sgrp, fmt):
     # type: (SettingGroup, srcgen.Formatter) -> None
     """
@@ -29,12 +55,14 @@ def gen_enum_types(sgrp, fmt):
             continue
         ty = camel_case(setting.name)
         fmt.doc_comment('Values for `{}`.'.format(setting))
-        fmt.line('#[derive(Debug, Copy, Clone, PartialEq, Eq)]')
+        fmt.line('#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]')
         with fmt.indented('pub enum {} {{'.format(ty), '}'):
             for v in setting.values:
                 fmt.doc_comment('`{}`.'.format(v))
                 fmt.line(camel_case(v) + ',')
 
+        gen_to_and_from_str(ty, setting.values, fmt)
+
 
 def gen_getter(setting, sgrp, fmt):
     # type: (Setting, SettingGroup, srcgen.Formatter) -> None
@@ -221,7 +249,7 @@ def gen_display(sgrp, fmt):
                 with fmt.indented('if !d.detail.is_preset() {', '}'):
                     fmt.line('write!(f, "{} = ", d.name)?;')
                     fmt.line(
-                            'TEMPLATE.format_toml_value(d.detail,' +
+                            'TEMPLATE.format_toml_value(d.detail, ' +
                             'self.bytes[d.offset as usize], f)?;')
                     fmt.line('writeln!(f)?;')
             fmt.line('Ok(())')