Fix compilation without std feature (#106)

src/hb/buffer.rs

@@ -1,6 +1,6 @@
 use alloc::{string::String, vec::Vec};
 use core::convert::TryFrom;
-use std::cmp::min;
+use core::cmp::min;
 
 use ttf_parser::GlyphId;
 

src/hb/ot/layout/GSUB/alternate_set.rs

@@ -1,7 +1,7 @@
 use crate::hb::ot_layout_gsubgpos::Apply;
 use crate::hb::ot_layout_gsubgpos::OT::hb_ot_apply_context_t;
 use crate::hb::ot_map::hb_ot_map_t;
-use std::convert::TryFrom;
+use core::convert::TryFrom;
 use ttf_parser::gsub::AlternateSet;
 
 impl Apply for AlternateSet<'_> {

src/hb/ot_layout_gsubgpos.rs

@@ -1,6 +1,6 @@
 //! Matching of glyph patterns.
 
-use std::cmp::max;
+use core::cmp::max;
 
 use ttf_parser::opentype_layout::*;
 use ttf_parser::{GlyphId, LazyArray16};