no-std support for monero-serai (#311)

* Move monero-serai from std to std-shims, where possible

* no-std fixes

* Make the HttpRpc its own feature, thiserror only on std

* Drop monero-rs's epee for a homegrown one

We only need it for a single function. While I tried jeffro's, it didn't work
out of the box, had three unimplemented!s, and is no where near viable for
no_std.

Fixes #182, though should be further tested.

* no-std monero-serai

* Allow base58-monero via git

* cargo fmt

coins/monero/src/wallet/seed/classic.rs

@@ -1,5 +1,10 @@
 use core::ops::Deref;
-use std_shims::{sync::OnceLock, collections::HashMap};
+use std_shims::{
+  sync::OnceLock,
+  vec::Vec,
+  string::{String, ToString},
+  collections::HashMap,
+};
 
 use zeroize::{Zeroize, Zeroizing};
 use rand_core::{RngCore, CryptoRng};

coins/monero/src/wallet/seed/mod.rs

@@ -1,25 +1,25 @@
 use core::fmt;
+use std_shims::string::String;
 
 use zeroize::{Zeroize, ZeroizeOnDrop, Zeroizing};
 use rand_core::{RngCore, CryptoRng};
 
-use thiserror::Error;
-
 pub(crate) mod classic;
 use classic::{CLASSIC_SEED_LENGTH, CLASSIC_SEED_LENGTH_WITH_CHECKSUM, ClassicSeed};
 
 /// Error when decoding a seed.
-#[derive(Clone, Copy, PartialEq, Eq, Debug, Error)]
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+#[cfg_attr(feature = "std", derive(thiserror::Error))]
 pub enum SeedError {
-  #[error("invalid number of words in seed")]
+  #[cfg_attr(feature = "std", error("invalid number of words in seed"))]
   InvalidSeedLength,
-  #[error("unknown language")]
+  #[cfg_attr(feature = "std", error("unknown language"))]
   UnknownLanguage,
-  #[error("invalid checksum")]
+  #[cfg_attr(feature = "std", error("invalid checksum"))]
   InvalidChecksum,
-  #[error("english old seeds don't support checksums")]
+  #[cfg_attr(feature = "std", error("english old seeds don't support checksums"))]
   EnglishOldWithChecksum,
-  #[error("invalid seed")]
+  #[cfg_attr(feature = "std", error("invalid seed"))]
   InvalidSeed,
 }