signature: Enable Ed25519 support for wasm32 targets.

[briansmith]

Ed25519 was disabled for WebAssembly due to some unrelated issues with
getting the X25519 code working in WebAssembly. Temporarily remove the
agreement API when targetting WebAssembly to work around those issues
in a way that lets us enabled Ed25519.

[codecov]

Codecov Report

Merging #1440 (80a5c0d) into main (08fcf4a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1440   +/-   ##
=======================================
  Coverage   93.02%   93.03%           
=======================================
  Files         127      127           
  Lines       18227    18227           
  Branches      195      195           
=======================================
+ Hits        16956    16957    +1     
  Misses       1237     1237           
+ Partials       34       33    -1     

Impacted Files	Coverage Δ
src/ec.rs	100.00% <ø> (ø)
src/ec/keys.rs	96.42% <ø> (ø)
src/ec/suite_b.rs	98.65% <ø> (ø)
src/lib.rs	93.33% <ø> (ø)
tests/agreement_tests.rs	90.74% <ø> (ø)
tests/ed25519_tests.rs	97.79% <ø> (ø)
crypto/curve25519/curve25519.c	99.77% <0.00%> (+0.07%)	⬆️

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[ivnsch]

Any updates? We're using this in an SDK https://github.com/manuelmauro/algonaut/blob/main/algonaut_transaction/src/account.rs#L35 and it would be great if it didn't break WASM compatibility..