mark core as nostd

* implement custom hashmap to back symtable
* pass in print and read callbacks to keep stdlib pure
* use core / alloc versions of Box, Rc, Vec, etc
* replace pow func with libm

Signed-off-by: Ava Affine <ava@sunnypup.io>

core/Cargo.toml

@@ -5,6 +5,7 @@ authors = ["Ava <ava@sunnypup.io>"]
 edition = "2021"
 
 [dependencies]
+libm = "0.2.8"
 # this one provides a global constant lookup table for simple
 # string escaping in the lexer
 phf = { version = "0.11", default-features = false, features = ["macros"] }
@@ -14,4 +15,4 @@ name = "flesh"
 
 # only turn this on if you set POSIX_LOAD_NAME at build time
 [features]
-implicit-load = []
+implicit-load = []