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.

cranelift/filetests/parser/rewrite.cton

@@ -9,7 +9,7 @@ ebb100(v20: i32):
     v9200 = f64const 0x4.0p0
     trap user4
 }
-; sameln: function %defs() system_v {
+; sameln: function %defs() fast {
 ; nextln: ebb100(v20: i32):
 ; nextln:     v1000 = iconst.i32x8 5
 ; nextln:     v9200 = f64const 0x1.0000000000000p2
@@ -23,7 +23,7 @@ ebb100(v20: i32):
     v200 = iadd v20, v1000
     jump ebb100(v1000)
 }
-; sameln: function %use_value() system_v {
+; sameln: function %use_value() fast {
 ; nextln: ebb100(v20: i32):
 ; nextln:     v1000 = iadd_imm v20, 5
 ; nextln:     v200 = iadd v20, v1000