build.rs: Add feature 'lowmemory' to reduce memory usage

Currently, this only set `ECMULT_WINDOW_SIZE` to 3 instead of 15.

Fixes #139.

.travis.yml

@@ -30,6 +30,7 @@ script:
   - cargo build --verbose --no-default-features --features="rand"
   - cargo build --verbose --no-default-features --features="rand serde recovery endomorphism"
   - cargo build --verbose --no-default-features --features="fuzztarget recovery"
+  - cargo build --verbose --no-default-features --features="lowmemory"
   - cargo build --verbose
   - cargo test --verbose
   - cargo build --release

CHANGELOG.md

@@ -1,3 +1,10 @@
+# 0.15.1 - ????-??-??
+
+- Add feature `lowmemory` that reduces the EC mult window size to require
+  significantly less memory for the validation context (~340B instead of
+  ~520kB), at the cost of slower validation. It does not affect signing, nor
+  the size of the signing context.
+
 # 0.15.0 - 2019-07-25
 
 * Implement hex human-readable serde for PublicKey

Cargo.toml

@@ -32,6 +32,7 @@ fuzztarget = []
 std = ["rand/std"]
 recovery = []
 endomorphism = []
+lowmemory = []
 
 [dev-dependencies]
 rand = "0.6"

build.rs

@@ -53,9 +53,13 @@ fn main() {
                .define("USE_FIELD_INV_BUILTIN", Some("1"))
                .define("USE_SCALAR_INV_BUILTIN", Some("1"))
                .define("ENABLE_MODULE_ECDH", Some("1"))
-               .define("USE_EXTERNAL_DEFAULT_CALLBACKS", Some("1"))
-               .define("ECMULT_WINDOW_SIZE", Some("15")); // This is the default in the configure file (`auto`)
+               .define("USE_EXTERNAL_DEFAULT_CALLBACKS", Some("1"));
 
+    if cfg!(feature = "lowmemory") {
+        base_config.define("ECMULT_WINDOW_SIZE", Some("3")); // This is the lowest accepted value by configure
+    } else {
+        base_config.define("ECMULT_WINDOW_SIZE", Some("15")); // This is the default in the configure file (`auto`)
+    }
     #[cfg(feature = "endomorphism")]
     base_config.define("USE_ENDOMORPHISM", Some("1"));
     #[cfg(feature = "recovery")]